Nerf Herder's opening theme tune, especially after it was re-recorded for the third season onwards. Awesome rock that represents the series campy and actiony yet somewhat serious tone.
Sarah McLachlan contributed "Prayer of St. Francis," which played over the scenes of Buffy and Dawn crawling out of the grave and Willow and Xander up on the hilltop in "Grave". Also beautiful and poignant.
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.
"Chinese Burn" by Curve serves as Buffy and Faith's hotblooded rave music in "Bad Girls" and it is memorably psychedelic; the lyrics also foreshadow Faith's downward spiral starting in that episode.
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.
The music playing right at the end of "Smashed" is stunning.
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. Amber Benson as well. She and Marsters both sing on some of the tracks on one of Anthony Stewart Head's albums.