Bleach or Nevermind?
And Nevermind for me has, 'Led Zeppelin 4 Syndrome' in that since every song has been so overplayed my whole life, it's shelf line is gone early, and theres no reason for me to listen to it unless it's a song not on the radio (Territorial Pissings).
That said, I still love In Utero and Unplugged, even though they've gotten their fair share of overplay, they haven't worn down as quick as Nevermind for me.
I really like Nirvana, not just for their music, but because Cobain so openly and passionately plugging his favorite banRAB in interviews has introduced me to:
The Pixies
The Stooges
The Shaggs
Killing Joke
The Vaselines
The Meat Puppets
Flipper
The Breeders
...and a bunch more. Might be the best band to get into to introduce yourself to the underground.
