My absolute favorite composer is Ennio Morricone, who has not done a lot of scifi/fantasy, but he did the phenomenal The Thing score for John Carpenter.

Aside from those already mentioned, I'd also add:

Akira Ifukube: Godzilla, The Mysterians, The Three Treasures, Battle in Outer Space, Atragon
Alan Silvestri: Back to the Future, Predator, Who Framed Roger Rabbit, Judge Dredd, The Avengers
Basil Poledouris: Conan the Barbarian, RoboCop, Cherry 2000, Starship Troopers
Bear McCreary: Battlestar Galactica (reboot), Agents of SHIELD, Constantine (TV), MotU: Revelations/Revolutions
Brad Fiedel: The Terminator, Fright Night, Johnny Mnemonic
Christopher Young: Hellraiser, Species, Virtuosity, Drag Me to Hell
David Arnold: Stargate, Independence Day, Godzilla '98, Paul
James Horner: Battle Beyond the Stars, Star Trek II-III, Cocoon, Aliens, Willow, Jumanji, Avatar
Lolita Ritmanis: Justice League/Justice League Unlimited, Teen Titans, Legion of Super-Heroes
Mark Snow: The X-Files, Millennium, Smallville, Birds of Prey (TV), Ghost Whisperer
Murray Gold: Doctor Who, Torchwood, The Second Coming
Shirley Walker: The Flash (19909, Memoirs of an Invisible Man, Batman TAS, Batman: Mask of the Phantasm, Space: Above & Beyond, Superman TAS, Spawn TAS, Mystery Men, Final Destination,