This is really hard... I don't think I can come up with a band who has released 5 or more albums that has never done something I didn't like... Steve Vai is the best example I can come up with, although I was never a huge fan of his last studio effort. Testamen't first 5 albums were great.
Other notables are...
Amorphis (never really put out total crap)
Nevermore (never really put out total crap)
Blind Guardian (minus "A Twist in the Myth")
Cryptopsy
Dream Theater (bumps in the road, of course)
Mercyful Fate
Symphony X (last 5 albums were good)
Yngwie Malmsteen (first 7-8 albums were good)
Shit, none of them are all that consistent obviously.
A Twist In The Myth is great when really drunk because it's a collection of the cheesiest/gayest songs ever written.
Dissection --> Reinkaos was bad
Even though that albums awful they almost deserve to be in lists for making some of the best songs ever within their genre.