Electric Wizard (Get Come My Fanatics... first, Dopethrone isn't as good as everyone makes it out to be.)
Sleep (Anything is good, but bear in mind that Jerusalem and Dopesmoker are essentially the same album, with Jerusalem being a truncated version, also they're one song albums.)
Melvins (Bullhead and Lysol.)
Orthodox (I only have Gran Poder, but I recommend that wholeheartedly, excellent drone doom.)
Sahg (Traditional doom fronted by members of black metal bands, and it works surprisingly well, get everything.)
Bongzilla (I only have Gateway, but it's pretty sludgy and awesome, get that.)
The Angelic Process (Not sure what to call these guys, bar drone doom, but you need to listen to Weighing Souls With Sand, because it is literally the thickest album you'll ever hear, you'll understand when you try it. Absolutely amazing.)
ISIS (All their old EPs before Celestial are pretty damn doomy and heavy as a motherfucker.)
Neurosis (I hesitate to dump them in a doom thread, because they're so much more than that, but they're essential. Start with Times of Grace, then go backwards for heavier Neurosis, and forwards for softer Neurosis.)