Christophe Beck's music for Buffy the Vampire Slayer was beautiful, although it was Rob Duncan who scored the final episodes. They also had awesome song choices: Michelle Branch's "Goodbye To You" used in conjunction with Giles and Tara leaving at the end of Tabula Rasa, Sarah McLachlan's "Full Of Grace" at the end of "Becoming Part 2", Aimee Mann's "Pavlov's Bell" in "Sleeper", and Angie Hart's "Blue" in "Conversations With Dead People" were all awesome, awesome, awesome.
Many of Beck's individual scores are simply amazing such as Close Your Eyes used throughout Season 2 and coined as the Buffy/Angel Love Theme for good reason, Slayer's Elegy from Season 3, and Sacrifice from Season 5's "The Gift."
The Final Fight has been used in commercials unrelated to Buffy as well.
Darling Violetta's theme tune for Angel was amazing (especially the extended version), and the music used in that series was pretty neat too. The Kim Richey song "A Place Called Home" used over the montage of Fred being remembered at the end of "Shells" is also beautiful and heartbreaking.
The most awesome Musical Episode ever, "Once More, with Feeling". Specifically, "Rest In Peace", "Under Your Spell", "What You Feel", "Bunnies", and "Walk Through the Fire".
All beaten out by Giles singing the "Exposition Song" in "Restless".
The music during the final battle in "Chosen" was truly epic.
"Passion". Giles walking up the stairs to what he thinks is going to be a romantic tryst with Jenny, only to find her corpse in the bedroom. All the while a beautiful rendition of "O soave fanciulla" from Puccini's "La Bohème" plays, one of the all time great love songs.
Get ready to weep copiously whenever 'Remembering Jenny' is played.
The 'Danse Macabre' in "Hush"? Creepy sinister music for the creepiest, most sinister episode of the show?
Sarah McLachlan's "Prayer of St. Francis" playing over the scenes of Buffy and Dawn crawling out of the grave and Willow and Xander up on the hilltop in "Grave".
The music playing right at the end of Smashed is stunning.
The Suite from "Hush".
How awesome is Beck's music? He'd left the show by season five, but they brought him back to score the final WB episode "The Gift."
This is all helped by the fact that both James Marsters and Anthony Stewart Head are talented musicians in their own right. This makes James Marsters' solo "Rest in Peace" all the more awesome.
Spike sings "My Way" twice in the episode "Lover's Walk". The first version exaggerates the melancholy of Sinatra's original, while the Triumphant Reprise at the end plays up the defiance of Sid Vicious's cover.
The music played by Oz's band, Dingoes Ate My Baby (which was provided by real band Four Star Mary) was consistently awesome. Special mention goes to "Violent", which they perform in "Band Candy".