I considered Averse Sefira, The Chasm, and Manilla Road as well. All exist within the upper echelon of metal, but none within the incomparably uppermost tier. The Chasm had a couple of mediocre releases, so did Manilla Road(plus one AWFUL album that shouldn't have really been released under the band's name in the first place). It is true that all three maintained quality (overall) throughout their careers, though.
Immolation is squarely in the crap zone by now. Third Gorguts starts getting goofy, fourth is junk. I haven't listened to anything but snippets from Bolt Thrower after ...For Victory either, but it doesn't seem worth much; like WAAGH said, great up until then. There are innumerable death metal bands that followed this course; the question is which bands both a) had many releases and b) never deviated. That disqualifies Bathory, Slayer, Mercyful Fate, Asphyx, Morbid Angel, Suffocation, Massacra, Pestilence, Candlemass, Burzum, Enslaved, and countless others. Same goes for Darkthrone, by the way, who are specifically interesting because their descent was so gradual - everything up to Transilvanian Hunger was excellent, Panzerfaust was pretty good, and then the next couple of albums were OK... and then...
Although I did just remember another band that might be worth including: Skepticism.