People make me cry when they say they're 80s thrash fans but don't mention Dark Angel at all.
No other band at the time even came close to the purity or speed of Darkness Descends. When every big name band (going by the "Big Four" media crap that wasn't true in the least) was leaning towards melodic tendencies, Dark Angel took thrash to its core. Pure speed, and almost no studio crap, minus a swell of guitar noise at the beginning of the album.
Dark Angel is fast, but they have their fast and slow parts like every other band. Except for Dark Angel, their slow parts are fast, and their fast parts are 'too fast, my ***.'
Who the hell else can hit 286bpm, or play well over 200bpm for 8 minutes straight?
Morbid Saint's Spectrum of Death is also another highlight of thrash, and like Dark Angel, will always be heralded as one of the best. They leaned towards a darker take on thrash, but were still amazing. Hell, despite how much I just raved about Dark Angel, these guys might be better. On the same level of amazing, at least.
Listen to both albums. Dark Angel's 286bpm song is Burning of Sodom. Look for downloads if you don't want **** quality; I couldn't find decent stuff online any where. Though the highest Dark Angel download I could find was 192kbps. Buy the albums for the real quality.
Now that I'm done with that, some other mentions:
Skeletonwitch: Fantastic mix of thrash aesthetic and melodic guitar work. They aren't imaginative at all as far as lyrics go, but it doesn't affect the quality in the slightest. Beyond the Permafrost is a fantastic album; it may seem to be all over the place, but Skeletonwitch know exactly what they're doing.
Destroyer 666: Destroyer may have a cliche name, but their music in no way reflects that. Their music can be described as a mix of thrash and black metal, with some death influence, but honestly, summing it up with genres downplays its quality. Instead, I'll simply say that they're one of the most talented metal bands out there who play the sickest extreme metal out there. The musicians aren't just technical, which seems to be all the focus with a lot of bands today; they're very skilled at creating their tense, foreboding, nihilistic atmosphere, with tight drumming, moody guitar, amazing leads (that aren't overplayed) and the best metal vocals I've heard to date.
Unchain the Wolves, Phoenix Rising and Cold Steel. . . for an Iron Age are all fantastic albums. The first has a darker, thrashier feel, while Phoenix Rising has fantastic, complex composition. On Cold Steel, however, D666 goes all out, with fierce aggression and atmosphere, tuning down the melodic-death-style complexity of the previous album to achieve pure assault. (I've yet to listen to their newest one.)
For an intro, I recommend giving Phoenix Rising a listen, as it's by far my favorite (though all three are fantastic). Listen to the other two if they spark your interest as well.
Agalloch: This band is hardly metal at all, but I feel obliged to mention them in this thread, as their music is just sublime. They're basically an atmospheric band with black metal and post rock influence, to sum it up as simply as I can. Agalloch are masters of atmosphere, however, creating album-long epics that are captivating and entrancing. It's the most beautiful music I've ever heard, and they're a band I recommend to everybody, regardless of their musical tastes.
They've released three albums since 1999, and each one is fantastic. I recommend the Mantle for an introduction to their music. Listen to it with a pair of headphones, and make sure to give the entire album a listen from the beginning, as this is a band that does not express itself through a single song. I'm not going to write near as much for this band, though, because I truly think their music speaks for itself. I've had the album for around two years now, and I still listen to it regularly; it seems to improve the more one listens to it.
Other mentions:
Krallice: a very experimental black metal sound, hardly fitting within the normal definition of the genre, but fantastic nonetheless.
Mors Principium Est: Solid melodic death, with progressive influence, especially on their album The Unborn (which is the one I recommend the most).
Dragonlord: Pretty legit USBM, can't say much more as I haven't listened to them enough, but give them a try; Black Wings of Destiny is the album I have at the moment.
Alestorm: The only pirate metal band that has actually acheived what their name suggests. Their music is epic as hell, though it's more for fun than to be taken seriously; great for parties and such. The musicians are not lacking in quality, however. Check out Captain Morgan's Revenge as an example. There may not have actually been Scottish pirates, but these Scots would sure as hell be fine candidates.
(The technical skill of the musicians is obviously not the focus of the music at all here, but if you want a taste of it, pay attention to the drum fill at the beginning of the album's title track).
Between the Buried and Me: The only progressive, [x]-core band that I really, really enjoy.
Machinae Supremacy: Awesome power metal. They use the Commodore 64 sound chip, too, which makes them even more win. Redeemer's the album I've listened to, but I've heard good things about the others as well.