Okay, here goes:
Tool:Ænima

Undertow

Lateralus

Those are the albums I'd recommend to get first, in that order.
Ænima is very versatile, shifting through most things you will experience with Tool, and when you have gotten comfortable with their universe you can shift over to Undertow, which is slightly more heavy in my opinion and not so experimental. And then if you have a need for more, take Lateralus, put it on the cd player, turn it up good and just sit there for the duration and experience how music can affect your senses, and even better, listen to it wearing headphones and you will continually hear new things.
I can put it on now and hear things i didnt hear 2 years ago, its as if its constantly developing itself, simply amazing.
And if you just dont like Tool because its too weird I will not recommend the newist album, 10.000 Days, but I'd still think you should give Opiate a try as its more raw and garage like, most people like it even though they arent into Tool.
Next up:
Metallica:...And justice for all

Master of Puppets

Ride the Lightning

Not much to say, most of you are without a doubt familiar with Metallica in one way or another, I've listed the albums i find to be the best, not necessarily in that order, but I think that was the order in which I got to know them, though I got Justice and Kill 'Em All at the same time, I just never really got to love Kill *Em All the way i love the other albums.
Other than that, I'll just mention a few of the other bands that i truly love listening to.
Guns N' Roses, the old one, not the newish shite that Axl got going *shudders* - But back from the Appetite and UYI days, that was some fine music.
Pantera, steady metal, easy to listen to though I dont toally fancy the vocals, but I'll live.
In Flames, whoah, awesome band, neat attitude, awesome vocals, awesome drums and neat guitar, if you havent already, do check them out, i can recommend The Jester Race for something a bit old and Clayman for something newer. Its in the metal category if you should be in doubt.
Other solid rock bands: Slash's Snakepit - 5 O' Clock Somewhere. Velvet Revolver - Contraband. Iron Maiden - Any album really.. i love the first and selftitled though. Almost forgot, Megadeth - Countdown to Extinction <- GET THIS!
If you are more into the grunge and depressed style of music, you should check out Alice in Chains, but if you are into the genre I dont think you could have missed them. Get a few albums perhaps, Facelift and Dirt might be pretty good starters.
And for "regional" music, all scandinavians, and perhaps even UK and German folks should check Kaizers Orchestra, a norwegian rock band, performing in their native tongue.
Cheers.