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I have no one favourite composer, so here are my top offerings:
Alkan
Ligeti
Shostakovich
Prokofiev
Liszt
Stravinsky
Berlioz
Telemann
In no particular order.
Stravinsky and Tchaikovsky would be my answers to this I guess. I'm far from a classical expert, so I'm sure I'm missing a lot of good ones, but I enjoy what I've heard of both those guys.