This is gonna be tough, but here we go, in no particular order:
Zero Hour-The Towers Of Avarice
Simply amazing. Incredibly technical, but retains a beauty that many technical bands lack. The lyrics are fantastic, and the vocal delivery by Eric Rosvolt is amazing. And on kind of a side note, my favorite album cover/art ever. As another poster mentioned above, if not for the "no repeats" rule, every Zero Hour album would be on this list, they are my favorite band
Faith No More-Angel Dust
A fantastic follow-up to the also good The Real Thing. They probably could have become a mainstream hit after TRT, but they decided to go the opposite direction, and for fans, the results could not be better. Mike Patton IMO is the most diverse and talented vocalist out there. So much variety can be found on this album.
Queensryche-Operation Mindcrime
One of the greatest concept albums of all time, Geoff Tate gives a legendary performance. Not a single weak track on the album
Dream Theater-Six Degrees Of Inner Turbulence
This was a tough choice, I had to put DT on my list, and it was between this, Scenes From A Memory, and Awake, all of which are amazing. I think it is their most progressive and experimental album, and that's why I chose it.
Symphony X-The Divine Wings Of Tragedy
Wow, what an amazing album this is! From the powerful, exciting opener, to the incredibly epic title track, this album pleases from start to finish. Symphony X has many great albums, but this is the one I would use to introduce potential new fans.
Mr. Bungle-Self Titled
This is the best album I have for freaking out my closed-minded friends. Incredibly creepy and sinister, with an odd blend of pretty much any genre you can name, including circus and porno music, I think this is the most varied album in my vast collection.
Spiral Architect-A Septic's Universe
I LOVE this album, one of the most technical that I have. This is the kind of album that just wont get boring, there are too many layers in there to discover.
Iced Earth-Something Wicked This Way Comes
I'm a pretty big Iced Earth fan, and I think this is their best album, especially the "something wicked" trilogy.
Into Eternity-Buried In Oblivion
Mixing Death, Black, Progressive, and Power Metal styles together, Into Eternity created an absolute masterpiece. I'm usually not a fan of the Death Metal Growls or the Black Metal Shrieks, but IE does them right, plus they mix in clean vocals, and the "choir" type vocals used in most power metal.
Porcupine Tree-In Absentia
A great concept albums about a serial killer, this is IMO, Porcupine Tree's best album, it was also the first one to contain consistent elements of metal, as well as their normal progressive rock styling. Songs like Prodigal, Gravity Eyelids, and Blackest Eyes make the hair on my neck stand up straight
Honorable Mentions:
Iron Maiden-Powerslave
Opeth-Blackwater Park
Tool-Lateralus
Fantomas-Suspended Animation