Jefferson Airplane - one of my favorite bands, but all of their albums have some crap on them (well, possibly not the first, but that one's just pretty boring), either because it's wilfully obnoxious (e.g. "A Small Package of Value Will Come to You Shortly"), irritatingly smug (e.g. "Plastic Fantastic Lover") or extremely tedious ("Spare Chaynge"). And that's not counting the last three pre-breakup albums, which have much higher ratios of garbage, and before you start going through the Jefferson Starship period and the solo work (although the first few Kantner and Slick albums are probably more consistent than the average JA album).
Juliana Hatfield - has a lot of albums, with her better songs spread pretty evenly across all of them.
The Beach Boys - I'm not even sure whether I love them, but I've been going through their discography to pick out the good songs, and there's a lot of terrible stuff (at least through Smiley Smile, where I am now). At least two-thirds of the upbeat songs are unlistenable, for example.
Amon Duul II - I love Made In Germany but it's got a fair amount of filler, and everything else of theirs I've heard is very inconsistent.
Fiery Furnaces, definitely. Grateful Dead? Kind of a unique case. Maybe Shirley Collins or Steeleye Span?