I'll throw in my 2 cents...
Dream Theater - I don't really dislike any of their stuff, but Awake gets played least.
ELP - Not having heard In The Hot Seat, I'd vote for Love Beach. But, Works 2 didn't do much for me either.
ELO - Discovery, without a doubt. My favorite being Face the Music
Genesis - Invisible Touch. I can tolerate We Can't Dance, but I don't listen to it but maybe once a year.
Gentle Giant - I haven't heard Giant For A Day, but I've been told its the worst by friends of mine that would know. I have the Twilight comp, Octopus, and Power&Glory and they are great.
Kansas - I haven't heard anything past Audio Visions, which gets my vote for the weakest, though not outright terrible.
King Crimson - In the Wake... doesn't get played much at my house. So, I'd have to say that I consider that one the weakest. I should note that I'm a yellow dog KC fan though.
Pink Floyd - This is a tough one, but I have to go with The Final Cut. But I do like most of it. I almost always laugh at Not Now John (so fuck all that...)
Rush - Roll the Bones. TJ will verify that I'm also a yellow dog Rush fan, but RtB hasn't been played since I bought it.
Yes - I haven't heard Open Your Eyes which gets the vote from friends of mine. So, I'd have to go with Tormato even though I like some of the stuff on there.
Zappa - I've got about 30 FZ records and discs, so I would consider myself a bit of a fanboy (but not in the league of Bice). Joe's Garage doesn't do a lot for me. The Yellow Shark hasn't really grown on me either. And that's about it...
Queensryche - The Promised Land. It only has a couple of redeeming tracks, but is easily the worst. Although, I haven't been that crazy about Q2K.
NP - Govt. Mule - Life Before Insanity