tech/prog has become an interesting and overused adjective for metal
because of the recent rise of popularity in "tech" metal, be it dm/bm/grind or whatever hybrid, theres been a lot of ultimately subpar but very "technical" metal lately. I would say that in general, the technicality of melody and harmony (the "progressiveness" of it, if you will) hasn't caught up with the technicality of structure-- theres a million bands playing confused, wandering, somewhat atonal metal lately with relatively banal, pedestrian interlocking parts. on the flipside, you have the instantly "digestable" tech metal bands that are entirely consonant and entirely boring (im thinking rhapsody, theory in practice, symphony x, that line of thought). I used to really like all this kind of metal indiscriminately, but after listening to it all, you realize only a handful is noteworthy.
so, echoing what some others have said, for some truly mindbending/expanding metal, id suggest
gorguts - obscura
(nothing comes close to it, though if you think something does, please tell me!)
morbid angel - FFttF
(this one rocks and rolls, has lots of energy, and some of trey's most creative riffing. all MA is pretty good as far as technicality/progressiveness goes. you see, theres two ways to think of something as prog/tech-- in the microcosm of the riff, or the overarching song structure. their riffs are always pretty inventive)
immolation - unholy cult
(this one has obscura flavorings in places but heavier, more straightforward)
zeni geva - freedom bondage or 10,000 light years
(not strictly metal, and they repeat/cannibalize themselves a bit, but great use of dissonance to form moving/emotive melodies)
