I comletely agree with Motorhead. If you listen to their box set, it sounds like one really long really kick-a$$ album fron to back.
I'd also have to say Slayer. Sure, they changed drummers and then changed back again. They also slow down then speed up, and have been tuning down for quite awhile now, but you know exactly what you're gonna get when you buy a Slayer record!
Hellacopters. They've become more classic-rock-esque and a little less punkish, but every song they write is gold, and they always sound like the Hellacopters.
Supersuckers. More country-ish here and there than the punk fury of the past, but never disappointing, and always the Greatest Rock-N-Roll Band in the World! (self-proclaimed... but I agree)
Electric Frankenstein have been consistenly excellent since they began over 20 albums ago!
Opeth. More prog and less black metal than the early albums, Orchid, Morningrise, My Arms Your Hearse, but each album is consistently mind-blowing and brilliant!
I used to feel this way about Tool, but the last album was so overblown and pompous that it just irritated me. Unfortunate.
Scissorfight. Even though we may never again get an album from them, they never disappointed. Older stuff was a little rougher, almost hardcore-esque, newer stuff is straight-up mountain-stoner-boogie. Outstanding!
Last and not least, Clutch. Again, older stuff was a bit more raw and hardcore-influenced, and they slowly morphed into more blues-based territory. I can honestly say that I equally enjoy every brilliant thing they've ever released!