I found this list incredibly hard to make final. It still isn't final. I could easily make it a "20 favourite" and still struggle to include everything I wanted. Certain episodes were added, removed, re-added and I'm sure I've missed episodes I love to pieces. So... in order or airing.
Season 1, Episode 12 - "Prophecy Girl"
It seems fitting that in the first season our first Big Bad would be a vampire. A strong and ancient vampire known as The Master. Prophecy Girl is a fairly low key season finale, but contains what has to be one of the most heartbreaking lines from our protagonist. Buffy is shown to be strong, resourceful, bubbly and light-hearted despite the horrors she must face as the Chosen One. So after we overhear her Watcher talking to Angel and discover at the same time she does that it's been prophesied The Master will rise and Buffy will die facing him, it's heartbreaking to hear Buffy act exactly like what she is... a young, teenage girl. She strops, tries to quit her destiny and then when she realises she can't, tearfully admits “Giles, I’m sixteen years old. I don’t wanna die.” In the end though she bravely faces The Master (after dying, of course) and kills him on top of the roof of the library. Looking the epitome of innocence in a long, white dress.
Season 2, Episode 6 - "Halloween"
A more fun choice this time. We're told fairly early on in this episode that Halloween is the one night of the year demons take off because they find it tacky. That soon changes when Giles' old friend Ethan Rayne shows up and sells half of Sunnydale cursed costumes that turns them into whatever they're wearing. It's the first time we see Willow really take charge after Buffy becomes helpless as an 18th century woman and Xander as the buff and brave army soldier comes in handy more than once after this episode. Seeing our Slayer helpless is something we rarely see and when Spike shows up it's a race against time for Giles to "persuade" Ethan to reverse the spell. An important episode for the canon of the show. Cordelia learns the truth about Angel, we see Giles as Ripper for the first time, it's the first appearance of Ethan, Willow shows her leadership skills and, perhaps most importantly, it significantly increases the seriousness of Buffy and Angel's relationship. Which becomes very important in...
Season 2, Episode 14 - "Innocence"
Our first appearance of Angelus. For over a season and a half we've heard tales of how brutal and sadistic Angelus was as a vampire, but as Angel we never see it. After Buffy and Angel make love for the first time Angel stumbles out of his apartment and we see his soul ripped from him reverting him to his old, un-cursed self. Thus begins one of the best arcs in Buffy history as Angel and Buffy play cat and mouse as Buffy learns to deal with the idea of killing Angel, the man she loves. In a very unsubtle and extreme way I suppose what we're shown here is how people can change after sex, especially men. But if we ignore that it's just terrifying to think someone you loved that much could instantly become the thing you're meant to fight. Things get no easier for the Scoobies; Willow sees Cordelia and Xander kissing and runs away distraught, Jenny reveals that her people were the ones who cursed Angel initially, Giles loses trust in Jenny for her deception, Giles finds out Buffy and Angel had sex which is what lead to Angel's "moment of true happiness". Oh, and Spike and Drusilla now have Angel on their side along with the newly reborn Judge who can be killed by "no weapon forged". Luckily, Xander still has his army knowledge from Halloween and gives Buffy a lovely gift of a rocket launcher.
Season 2, Episode 17 - "Passion"
When Buffy went for the heart it hit it. Hard. This starts a long line of character deaths that actually have meaning in the show. Joss Whedon wanted this episode to show that Angelus isn't just a bit mean, that he doesn't just taunt, that he's evil. 100% sadistic. Just as Jenny and Giles start to rekindle their relationship, just as Jenny starts working towards redemption and working out how to restore Angel's soul we have Angelus stalk her through a dark and deserted Sunnydale High... and snap her neck. Not only does he kill her in an incredibly brutal way, he poses her body for Giles to find. This leads to Giles trying to take on Angelus, Spike and Drusilla single handedly. And he does quite well for a while, in the end Buffy shows up and saves him stating she can't do it without him. This marks the point Buffy realises she can kill him and needs to. The episode is made all the more bitter as we watch the disc containing the spell to restore Angel slip between Jenny's desk and cabinet, lost. This episode also has a chilling and beautifully poignant voiceover by Angelus which just adds to atmosphere. The scene that kills me most? Angelus watching Buffy and Willow find out about Jenny Calendar's death from outside the Summers house and the happiness in his face as the girls break down.
Season 2, Episode 22 - "Becoming Part 2"
So your ex-boyfriend turned evil and is trying to suck the Earth into Hell... you're also a fugitive from the police because they think you killed the Slayer that was called when you died last year. Your friends are hospitalised and/or scattered after you fell for a trap and one of your arch enemies wants to team up with you to stop his "grandsire" from destroying the world. Yup. At the start of the season two finale things are bad. Very bad. Giles is being tortured by Angelus, Willow is in a coma, Xander has a broken arm and the world is about to be sucked into a hell dimension. Kendra is also dead after Drusilla slit her throat. I love this episode though and it has some major significance as far as the canon of the show goes. We see Willow working some major magic in restoring Angel's soul (just too late), it's the first hint we get that Spike may become an ally (though it takes him a few seasons) and Joyce Summers finally finds out about Buffy's true identity (though it's revealed in the episode Normal Again that Buffy told her parents when she was first chosen). Oh, and Buffy and Angelus have a sword fight which is just cool, really. But the worst moment is the terrible timing of the spell Willow's working. The gate is already open, so only Angel's death can seal it... but Buffy gets to look into the confused and lost eyes of the man she loved, fully restored, gets to have one last kiss before running him through with her blade and saving the world.
Season 3, Episode 12 - "Helpless"
I think that Helpless is quite an "out there" choice in a list of Buffy's best episodes. It's a simple story of a father/daughter relationship between two people who're not at all related. We discover that on a Slayer's 18th birthday (if they live that long) they're put through a test of endurance to see how they cope without their powers. With only their training to help them the Watchers Council weakens them without their knowledge and puts them in a life or death situation. We see Buffy's father bail out on their yearly birthday bonding session at an ice show and Giles betray Buffy's trust by poisoning and weakening her. Quentin Travers arrives in town with an insane and drugged up vampire and along with two fellow Watchers seals up an old hotel where Buffy will have to do battle, win or die trying. Things go awry, however, and one of the Watchers is turned and one killed. So when insane vampire Kralik escapes and kidnaps Buffy's mother she has no choice but to face him. Our heroine succeeds in saving her mother through the cunning use of holy water and Giles shows up just in time to slay the final vampire for his weakened Slayer. Quentin fires Giles because he loves Buffy as a daughter and is no longer willing to play his part for the council. It's touching and Giles tending Buffy's wounds at the end is beautiful. They both need this relationship, and they'll always have it. This also marks the start of Buffy's estrangement from the council and eventual rebellion which goes on until season 5.
Season 4, Episode 16 - "Who Are You?"
I'm over halfway through this list and I've not even mentioned Faith. Here's a quick summary... Faith was called when Kendra died. She arrived in Sunnydale and at first, Buffy and Faith didn't get along. Faith's style was a lot different from Buffy's. They were opposites, but for a lot of season 3 they got along and were allies. Faith accidentally killed a human and afterwards was well and truly on her path to the Dark Side. Season 3 Big Bad, Mayor Wilkins, treated her kindly and with respect and after trying to kill Angel, Buffy puts Faith in a coma. It's not until the first of this two-parter This Year's Girl that Faith awakens to find out Mayor Wilkins is dead and his Ascension failed. Faith does eventually seek redemption after a stint in Buffy spin-off Angel and helps save the world from The First in season 7, but this episode marks the first point in Faith's eventual change. In Who Are You? Faith manages to switch bodies with Buffy and proceeds to ruin our Slayer's life once and for all. She allows Buffy to be imprisoned as she looks and sounds like Faith, screws her boyfriend and blows off her friends. Buffy breaks free and eventually corners Faith as she attempts to save a church full of people from vampires. Faith slays the vamps then she and Buffy brawl ending with Faith violently beating on her own body, Buffy switches them back and Faith runs to Los Angeles to kill Angel. Sarah Michelle Gellar and Eliza Dushku do brilliantly playing each other and rarely miss a beat in each others shoes. It's an unsettling body swap scenario, but is ultimately key to Faith's redemption.
Season 5, Episode 12 - "Checkpoint"
The Watchers Council are back. This means trouble for our protagonist. Buffy fired the Watchers Council a long time ago and they aren't going to forget it. Buffy needs their help, however, as season Big Bad, Glory is just too powerful and they have no information on how to stop her. The council will only help her if she jumps through their hoops and proves herself worthy. This is another fairly low-key episode. It has humour and there's not a huge amount of action. As the council interviews each of Buffy's friends in turn we can see that it's not going well. Willow and Tara, although incredibly powerful witches, have had no formal magical training and tests of their proficiency. Xander is human, a civilian and Anya is an ex-demon who's constantly worried they'll discover her secret and kill her. The council is unimpressed, but after a tense run in with Glory at her house Buffy realises something... she has the power and she's the one in control. Buffy loves a good inspirational speech and the one at the end of Checkpoint is great. She tells the council how important her friends are to her cause, she makes them listen and stands up to them. They eventually agree to work for her and to reinstate Giles as her Watcher with retroactive salary from the time he was fired. Oh, and the episode ends on the twist reveal that Glory isn't a demon... she's a god.
Season 6, Episode 8 - "Tabula Rasa"
This comes right after Once More, With Feeling where we saw Tara decide to break up with Willow for messing with her memory and using too much magic and Giles decide to leave as Buffy was relying on him too much. Can't get much worse, right? Wrong. Willow tries to go without magic and fails miserably, she attempts to use the same spell she used two episodes before on Buffy and Tara to make them forget the bad that happened the last time she used magic on them. Not very sensible, really. The spell goes awry and wipes all of the Scoobies memories of who they are. "Tabula rasa" actually means "blank slate" and that's what becomes of our heroes. They work out to a certain degree who they are through ID etc., but a very literal loan shark is after Spike and hilarity ensues as the gang fights and runs from vampires and demons. What is at heart a fairly hilarious episode both starts and ends very seriously. Giles is leaving for England and Tara and Willow are done. This is the first real glimpse we get of Willow's addiction to magic which becomes very important as the season progresses. Funny and heartbreaking are words that could be used to describe a lot of Buffy the Vampire Slayer and this episode epitomises that best of all.
Season 7, Episode 7 - "Conversations With Dead People"
This is a strange episode of Buffy. There's no title sequence for one and none of the main characters interact. Xander is also missing throughout. Instead we get four different stories shown in real time. Buffy talking to a newly sired vampire, Willow talking to the ghost of a girl who died in a previous episode supposedly talking on behalf of the murdered Tara, Jonathan and Andrew from season 6 returning to Sunnydale after they ran from Dark Willow and Dawn trying to talk to her mother, but being stopped by an evil force. Each story is told independently and each has a different tone. Buffy's is deep and emotional, but still fairly light hearted, Willow's is powerful and talks of her love for Tara and how sorry she is for her actions in season 6, Jonathan and Andrew appear on a quest for redemption, but unfortunately for Jonathan, Andrew is being controlled by their old friend Warren from beyond the grave. Dawn's story is where the real terror of the story lies, as she tries to save her deceased mother from an evil spirit. This episode is great for arc significance. We see the Hellmouth uncovered and Andrew trying to open is with Jonathan's blood. We discover a newly ensouled Spike is still killing and worse, siring. We hear Joyce Summers tell Dawn that Buffy won't be there for her. And Willow discovers she's been played all along by this seasons Big Bad, The First. It's a hard episode to describe, it's very different, but contains the same humour and drama you'd expect from Buffy. The best way to understand it, is to watch it. The same for every episode I've talked about.