Angel's eponymous episode in season 1, and The Reveal of his vampire nature.
"Prophecy Girl": The Season 1 finale had Buffy's death (albeit a temporary one).
Season 2
"School Hard": Spike makes his first appearance, and leads you to believe that he'll just be another Monster of the Week... and then he kills the Annointed One.
"What's My Line": Buffy's death resulted in another slayer being born, and Drusilla returns to full health.
"Surprise"/Innocence": These episodes have one of the biggest plot twists of the entire series. Angel and Buffy get intimate for the first time, but since this resulted in Angel experiencing a moment of true happiness, he loses his soul and becomes Angelus again. Buffy is unable to turn him back. Also, Jenny Calendar tells Giles and Buffy why she came to Sunnydale, and neither of them can handle it very well.
"Becoming: Part 2": Where to even begin with this episode? This Season 2 finale was one of the biggest Drama Bomb episodes in the entire series. The biggest moment is when Angelus' soul is restored, but Buffy has to kill him in order to defeat a demon who he summoned. In addition, Joyce learns about Buffy being a vampire slayer, Buffy gets expelled and runs away from Sunnydale, Kendra dies, and Willow dabbles in the dark arts for the first time.
Season 3
"Faith, Hope, and Trick": This episode wraps up most of the plot threads left from Season 2. After Kendra's death, Faith becomes the new second slayer and joins the Scoobies. In addition, Buffy returns to school and tells everyone the truth about Angel's death, unaware that he suddenly returned from hell.
"Lovers Walk": Spike appears for the first time this season, only to mess up everyone else's love lives to save his own with Drusilla. Buffy and Angel split up, and Cordelia walks in on Willow and Xander kissing.
"Helpless": Giles gets fired from the Watchers Council
"Enemies": After the events of the last three episodes, Faith has become truly psycho, plotting to destroy the world. It also seems like Angelus returns, but it ends up being a ruse.
"The Prom": Buffy and Angel break up, and he announces his plans to leave to Los Angeles. These events ultimately kickstart Angel.
"Graduation Day: Part 2": Similarly to "Becoming", this season finale was another massive Drama Bomb episode. For starters, the Mayor is defeated by Buffy blowing up the entire school, Angel leaves to Los Angeles, and the seniors graduate to go to college. In addition, Buffy quits the Watcher's Council causing Wesley to get fired, Anya leaves in fear of the Ascension, Principal Synder gets eaten, and Buffy puts Faith in a coma.
"Pangs": Spike makes the transition from reoccurring villain-of-the-week to member of the main cast.
"Hush": Despite being almost entirely silent, this episode has the pivotal moment of Buffy and Riley learning about each other's secret identities. In addition, Tara makes her first appearance.
"The I In Team": Buffy joins the Initiative, but when she learns about Room 314, Walsh tries to get rid of her. However, her plan fails when Buffy survives and reveals everything to Riley, causing him to walk out on the Initiative. Meanwhile, Walsh's secret project 314 is revealed to be a Frankenstein Monster named Adam, who stabs her in the chest, cementing himself as the Big Bad for the rest of the season. Wham!
"New Moon Rising": Oz returns having learned to control his werewolf instincts, but he leaves again when Willow opens up about dating Tara. In addition, Spike teams up with Adam, and Riley quits the Initiative for good.
"No Place Like Home": There's a reason why Dawn just suddenly appeared out of the blue. It turns out that she's actually a mystical object know as 'The Key' and was inserted into reality during the season premiere. Also, a mysterious woman in red, later revealed to be a hell god named Glory, is pursuing Dawn.
"The Body": Joyce dies. Often considered to be one of the biggest wham episodes of the show.
"Tough Love": Glory thinks that Tara is the Key, and mind-wipes her when she learns that she isn't, essentially reverting her to a child. Willow snaps and tries to use the Dark Arts to exact her revenge, which ultimately results in Glory learning that Dawn is actually the key.
"Spiral": Ben turns into Glory in front of everyone, slaughters all of the Knights of Byzantium, and takes Dawn. The episode ends with Buffy left in a catatonic state.
"The Gift": While the Scoobies manage to kill Glory/Ben, the Doc somehow comes back to life and opens the dimensional portal. However, Buffy sacrifices herself in place of Dawn to save the world. Anya and Xander also get engaged.
Season 6
"After Life": Buffy wasn't just resurrected, she was dragged right out of heaven.
"Once More With Feeling" has a big moment about pretty much every character. Giles decides to leave, Dawn is becoming a kleptomaniac, Buffy reveals that she was in Heaven, Tara learns that Willow brainwashed her to avoid an argument about her reliance on magic, Buffy and Spike kiss, and Xander and Anya start to express fears about getting married. And in the next episode, Giles leaves to England, Buffy and Spike make out, and Tara breaks up with Willow.
"Hell's Bells": Xander breaks up with Anya on the day of their wedding.
"Seeing Red": This episode ranks as one of the biggest Drama Bomb episodes in the series. Warren uses an orb to make himself more powerful, leaving Jonathan to tell Buffy on how to defeat him. However, Warren manages to escape, leaving Andrew and Jonathan to be arrested, with the latter learning that Warren was planning on betraying him anyway. In addition, Spike tries to convince Buffy that she loves him by initiating sex that she doesn't want which results in an attempted rape, and leaves Sunnydale after reflecting on his behavior. But the truly shocking moment is at the very end of the episode, when Warren suddenly appears, shoots Buffy, kills Tara, and leaves. The episode ends with Willow's eyes turning red with anger, as she becomes Dark Willow, the Big Bad of the rest of the season. This moment in particular is one of the most iconic and devastating plot twists in TV history.
The entire Dark Willow arc:
"Villains": Willow becomes Dark Willow and kills Warren as revenge, intending on going after Andrew and Jonathan next.
"Two To Go": Buffy, Xander, and Anya break Andrew and Jonathan out of prison to protect them from Willow, who almost kills Buffy and Anya if not for Giles returning.
"Grave": Willow traps Buffy and Dawn in a pit, rendering them unable to stop her from destroying the world. Xander ends up being the one to make Willow turn good again, only for her to start breaking down in tears. On top of that, Dawn finally proves herself to Buffy, Andrew and Jonathan flee to Mexico, and it turns out that Spike left to Africa so he could get his soul back. And the episode ends with him doing just that.
Season 7
"Beneath You" Spike's soul is revealed to a horror-stricken Buffy.
"Selfless": Anya decides to sacrifice herself to revert a massacre that she had accidentally caused. However, D'Hoffryn kills Halfrek instead, and forces Anya to become mortal again.
"Conversations With Dead People": The First Evil makes itself known to Willow, Dawn, and Andrew by disguising as Cassie, Joyce, and Warren respectively. It convinces Andrew to kill Jonathon, terrifies Dawn, and tries to get Willow to kill herself. In addition, Buffy opens up to a vampire only to learn that, despite having a soul, Spike is still siring people.
"Never Leave Me": The Watcher's Council is destroyed and a new, stronger vampire is introduced.
"Dirty Girls": Faith arrives in Sunnydale to help Buffy, and Caleb stabs Xander's eye out.
"Empty Places": After a series of repeated failures, the group kicks Buffy out of the house and makes Faith leader instead.
"Chosen": This is the Grand Finale, so it was definitely a big episode. Willow alters the magic of The Chosen One, such that it applies to all possible (i.e. "unactivated" as well as "activated") vampire slayers in existence. The Scoobies team up to defeat the First Evil, with Spike sacrificing himself to destroy the Hellmouth (albeit all of Sunnydale as well). Anya and Amanda also die during the fight.
