I'd have to say that Nirvana were one of those rare bands who never did anything wrong. Even Alice in Chains had a few duds, Pearl Jam gets a little boring at certain moments, and Soundgarden flat out blew a handful of times, but Nirvana had a pretty perfect track record. I mean even their early demo material was pretty awesome.
Alice in Chains are still my favorite though. Dirt is permanent Top 10 material right there.