Also check out
DEATH - crazy ass shythm and guitar solos
The Sound of Perseverence is good.
MORBID ANGEL - crazy ass rhythm and gutiar solos, just pick an album and choose a few songs.
SLAYER...diabolus and below, perferably something off of Reign in Blood, all that is good fun.
METALLICA...black album and below
MEGADETH before risk...just about anything learn all of Rust in Peace if you want to learn some awesome guitar.
MARTY FRIEDMAN - Some of his solo stuff is pretty insane though not much between his first and most recent endevours. Dragons Kiss is by far his best solo album learn something off of that.
NEVERMORE - heavy rhythm bad ass insane gutiar solos, THe Politics of Ecstacy, or Dead Heart in a Dead World rule and would definately be a good addition.
JASON BECKER - insanely melodic shredder, seek out
Perpetual Burn - the song Perpetual Burn will defiantely give you a run for you money.
MALMSTEEN - why not? Too many people already use the same ideas he does, do the same as them. If anything learn his Concerto Suite for the Electric guitar, since it has all his favorite repeated licks from all of his solos throughout.
ICED EARTH - good for insane triplet rhythms, not so much killer guitar solos. Learn Stormrider.
SYMPHONY X - More insane guitar work, probably kill your hands just as fast as Malmsteen, Becker, Loomis, or Azagthoth...
KRISIUN - definately brutal death metal, super fast rhythm or all kinds and whack ass totally off time guitar solos.
Other than that, I'd say there is a senseless NU-metal old-metal debate going on here. I say, play what you like. Learn whatever you want to learn how to play, however if you play the same song for 5 years and go, "I can play gutiar." The only guitar you will be playing is that song. Learn things outside of your skill level, it doesn't hurt you if you are able to play songs well beyond what people will hear coming from you in the end, if anything it will make it easier on you as a guitarist. Hell I think all you'd have to do is get a chord progression and detune and throw in some angry vocals about how you hate your parents, your life, your ex girlfriend and you've got a nu metal band. My opinion aside...Eh...Any of those bands well are good for improving your level of skill. Most importantly have fun with it.
