Mahler I love because he took the full symnphony to it's limits. he created music that relied on so much depth of instrumentation and intensity. His symphionies are over the top and fabulicious.
Beethoven is a dope moterfucker, one of my favorite, and his concertos are suiblime. I am a huge fan of concertos where the composer works to create a supremely technically challenging yet beautiful pioece of music for the soloist to play. Here is Beethoven's Triple concerto (violin, cello, piano) with YoYo Ma on cello, Itzhak Perlman on violin and Daniel Barenboim (also conducting the Berlin Philharmonic) in a fantastic performance.
http://www.youtube.com/watch?v=e01FtIEeXEU
I love so much classical music.
I love Bartok, whose quartets are some of the most dissonant sonic youth-esque classical music I have ever heard. bela Bartok rules all.
here is a movement from a bartok string quartet
http://www.youtube.com/watch?v=IS247Y9BSo8
I love Wagner. His Operas are epic and massive and complex and beautiful and demanding and just exquisite. Here is palcido Domingo as Parsifal
http://www.youtube.com/watch?v=O3FlSLRzoGY