Agalloch, with no bad albums, at one point I would have said The Grey is bad, but it was more of an acquired taste EP.
Sculptured, with no bad albums out of a whopping three. You could argue that the lyrics on The Spear of the Lily... are cheesy, but not offensively so, and the music is good on all three albums. Assuming you like avant-garde atonal jazz-influenced metal.
Clutch might make the list if we are ignoring production issues, Strange Cousins From the West has not nearly enough bass, but would sound great if they were playing live. They've had something like eight albums, I'm probably wrong on that.
Out of those three, one thing is consistent, and that is that the music isn't. The music is always good, but they change up the style. Don't know if that helps or hurts consistency of quality.
Also, Down with three albums, there probably should be some sort of lower limit on how many albums a band has to have before they count as consistent.
Maybe CoC and Crowbar, but I don't know much about their earlier stuff, and a couple of Crowbar's albums are kind of boring.