In no particular order of preference:
Guns n' Roses: Appetite for Destruction
This is the first CD that I ever bought, back when I wasn't even 10. It's the sound that started it all for me: Rock, Metal, Rebellion, Guitars, you name it, this album has it! Still listen to it eveyry once in a while and I'm still awe-struck by Slash's playing...
Metallica: The Black Album
This is the disc that got me to pick up the guitar. While I went on to discover (and love) all earlier works of Met, this album came to my life at the right moment. I might have started playing the guitar even without this one, but I'll never know. It was this one that made me do it.
Dream Theater: Awake
This CD got me into Progressive music. Nuff said.
Opeth: Still Life
Do I really need o give any reason here?
Kyuss: Blues for the Red Sun
Didn't so much touch me for the music, but for the experience surrounding it. I used to play in a Black metal band, and we used to get stoned before rehearsals, listening to this disc over and over again. It's nohere near anything Black Metal related, being Stoner Rock, but it's just got these MASSIVE walls of riffs.
Dream Theater: 6 Degrees of Inner Turbulence
When this double-disc came out, me and 2 friends made a deal that we'd listen to the 2nd disc's 42-minute song only together for the first time, and it had to be while under the influence of magic mushrooms. Needless to say, the experience was magic.
Ulver: Bergtatt
I've been listening to Black Metal a while before, but when I first heard this CD, it was like a whole new level of consciousness. This, to me, is the epitome of what I like in Black Metal. I don't listen to BM anymore, but Ulver still gets put on some times...
Iron Maiden: The Number of the Beast
Hey, c'mon, it's Maiden! I could have put any one of the first 7 of their albums here on the list, but this was the first one I bought. There's so many gems on this disc...
Pink Floyd: The Dark Side of the Moon
Floyd fans are often said to be divided between "DSotM"-fans and "The Wall"-fans. Well, I'm one of the former. I love PF, but this CD just stands out for some reason I can't explain
Led Zeppelin: The Early Days / The Latter Days
One of the noumerous "best-of" CDs, this one got me into the Zep, and since it's a doube-disc, it's got pretty much all the good songs covered. There are Zep songs that I love and that aren't on this set, but all the ones that are are just pure genious. This CD made me realize that, except a very few rare cases, EVERYTHING released today pales in comparison to what has been done at the end of the 60s, the 70s, and the early 80s
Damn, has that been 10 discs already? Well, I had to limit myself, but this sums it up quite well, I think.