Bolded are ones people have already posted (that are really good)
!T.O.O.H.! - if you never listen to another grindcore song in your life, listen to this one. Whoever uploaded it messed with the levels a bit, though.
Agalloch - Caspian, you've obviously never gotten stoned and driven on a winding mountain road during a snowstorm listening to
Ashes Against the Grain.
Anata - these guys are...harmonic death metal? Kind of a dueling-guitarist type deal.
Arch Enemy - these guys are pretty famous now, most of their stuff is pretty solid (even the albums with Johan Liiva, I don't care if the dude was shite for live shows and a prick)
Arcturus -
The Sham Mirrors is what you want to check out
Arsis - check out the song 'A Diamond For Disease', honestly it's the only song of theirs I like, but you'd probably like some of their other stuff. That song is too long for youtube, though.
Augury - probably my favorite band. Metal for comets to listen to during their trips through the void.
Baroness - I just found this fanmade video, it's pretty sweet.
Cynic - fuck all the haters. Not that there are any on this board. Proto-tech-death-jazz-fusion.
Dethklok - needs no explanation
Genghis Tron - I would just like to point out that if brokenCYDE and Attack Attack! had an antithesis, it is these guys. No crabcore here.
In Flames - any of their albums up to Whoracle, really driving melodeath/viking type stuff. After that they kind of stopped being good.
The Jester Race/Black Ash Inheritance still hasn't gotten old.
Kronos - see
Colossal Titan Strife. Probably the most deathy band I'm recommending.
Lykathea Aflame - the second most deathy. We will probably never hear music like theirs again.
Mastodon - this board is kind of love/hate about them. I like them. Whatever.
the Pax Cecilia - if Opeth wasn't so wanky, this is what they'd sound like. You'd probably like Opeth, too.