21st century Breakdown. I like early Green Day more than the more recent Green Day, but American Idiot is still either my second favorite or MOST favorite Green Day album. I seriously like every song on that album, and something about the story resonates with me.
21st Century Breakdown comes around. No epic songs a la Jesus of Suburbia, the longest is fucking 21 Guns. No clear storyline. I'm starting to think it was just Billie Joe writing down his ideals and saying "Oh wait, that's just how Christian feels!". A few really crappy songs, and a lot of mediocre ones. Christian's Inferno and See the Light flat out sucked, which is awful because Whatsername (American Idiot) is probably my favorite finale to any album ever. The only tracks off that album I DID really like were the title track, Before the Lobotomy, and East Jesus Nowhere. The rest of them were just kind of "meh". Basically, my least favorite Green Day album.
Also, What Hits!? from the Red Hot Chili Peppers. Simply because I heard Knock Me Down and knew Under the Bridge and loved them both, so I was hoping the rest of the album would be something like that. For some reason, I didn't really like the rest of it, and it was a Greatest Hits. At least Stadium Arcadium is awesome, although impossible to listen all the way through.
EDIT: Majorly ninja'd with 21st Century Breakdown