My favorite is the Ligeti. It was among the first pieces of "classical" music I was exposed to, obviously in the movie 2001:a space odyssey. I must say it scared the hubris out of me.

I also like the Mozart. It is almost the only Mozart I can relate to as it is not overloaded with Alberti bass and appoggiaturas. (Sorry to all you who worship Mozart).

Finally at the risk of being run out of this forum on a rail, I actually kind of like Andrew Lloyd Webber's Requiem. Don't misunderstand me. I don't think it's a masterpiece, but I enjoy the atmosphere and I always think there's room for a synthesizer in an orchestra. Hearing Placido Domingo rocking out in the Hosanna is kind of novel. I do not currently have this in my collection however and would not admit it if I did.

no worries, same thing happened to me for saying that Phantom was a cornerstone in contemporary music.

One which hasn't been mentioned yet is Cherubini's C Minor requiem, which is very impressive indeed. He also wrote one in D Minor, which I've heard but honestly don't remember all that well. If, by any chance, you ever get to hear Cary Ratcliff's Requiem, do so. It's for chorus and chamber ensemble (about five or six instrumental players, if I recall correctly), and is thus quite intimate emotionally. Truly a lovely piece, which I've heard live but never seen recorded.