Joss has always been big on promoting indie/underground music. The "performances" themselves on Buffy was pretty hit or miss. But some of it is worth noting.
Branch was good of course, as was Aimee Mann. And The Breeders and Dandy Warhols both made appearances.
Angie Hart of Splendid made three or four appearances.
I think Nerf Herder (who wrote the main theme) made one appearance.
And I used to be a big fan Cibo Matto. I thought "Sugar Water" was used really well in "When She Was Bad."
Bif Naked's appearances in season four were all pretty good too.
Story wise, there was Four Star Mary who did all the music for Dingoes. They're okay, but nothing special. I do, however, really like THC who did all the music for Veruca's band. The real singer was the chick playing the bass behind Paige Moss.
I think the music used for the soundtrack was much better. The McLachlan stuff goes without saying, but there's others I really liked.
Both songs from Union Station are excellent, even if bluegrass country seems like and odd fit. "It Doesn't Matter from "When She Was Bad" and "That Kind of Love" from "Entropy." Interestingly, Alison Krauss fairly recently did a duet album with Robert Plant of all people.
Probably my favorite of the whole show was Dollshead's "It's Over, It's Under" from "The Harsh Light of Day." from their album
Frozen Charlotte. I love the album. Admittedly the lyrics are kind-of that 90s self-loathing emo. But every song on it has a great hook and catchy melody. The music is kind of a blend of trip-hop, techno, and British grunge with a pop finish. I definitely recommend it.
However, one of my favorite musical choices from Buffy is when "Wild Horses" plays at the Prom.
That was The Sundays. If you're interested, the song is on their '92 album,
Blind.