For classical stuff (although I really do hate that term to describe anything before around the 1930's) then I'd have to go with Beethoven, Saint-Saëns, Stravinsky and Mahler.
For more modern composters then Jerry Goldsmith, Michael Giacchino, Delia Derbyshire (a really underrated 1960's electronic composer, and cute as all hell as well), Jeremy Soule does excellent stuff with his sampled orchestra and Philip Glass.