Transatlantic.
Yes, yes, I know they ONLY have three albums out, but they're flawless to me. Each album is astonishing, their playing and performances are awe-inspiring, and the music just never gets old for me. I can pop in any of their 3 albums at any time and be excited for a wild symphonic prog ride!
As for any other bands...hmmm. It'd odd, a lot of my favorite bands have at least one song or album that just doesn't stand up to the rest, like Yes, Genesis, Rush, Dream Theater, The Flower Kings, Spock's Beard, Marillion, etc. etc... each of those bands have had great highs, but some particularly mind-boggling lows as well.
I suppose, if you're a band with 3-5 albums, you're more likely to fit into this category.
-Marc.