My personal favorite genres are:
Fantasy - Sword & Sorcery, Appendix N oriented, & Dungeon Crawl stuff. Not into High Fantasy. Examples: DCC RPG, OSE, Black Sword Hack
Super Heroes - Everything from Golden/Silver Age to Modern, Pulp Hero type stuff included as well. Examples: Mutants & Masterminds, Champions, Marvel Heroes/FASERIP, Mayfair's DC Heroes
Espionage/Action Movie - Spies, Ninjas, Mercs, preferably all mashed up together into one. Examples: Ninjas & Superspies, Mercenaries, Spies, & Private Eyes, Top Secret/SI
Horror - Lovecraftian/Cosmic Horror - Call of Cthulhu, Sci Fi Horror - Mothership
Science Fiction - Space Opera & Cyberpunk. Examples: WEG Star Wars, Stars Without Number, Traveler, Cyberpunk 2020, Shadowrun, Cities Without Number
Post Apoctalyptic - Gamma World, Mutant Crawl Classics
Comedy - Ghostbusters/Spooktacular, Paranoia