My main game is Werwolf the Apocalypse, although I find all the original WoD games are pretty good. However with Werewolf I find a strong connection in the idea of trying to save the world, well in their eyes, against almost unbeatable odds. With a good mix of spirituality, horror and social gathering thrown into the mix.
As a StT I ran septs as they they where described, with politics that can easily match any Vampire game, and I loved the epic feeling the game gave as you fought often really long campaigns that could last for as long as you pretty much wanted.
Other faves have included Dark Sun, perhaps my fave D&D setting of all time, don't know what it was but something just clicked with me and that game.
If I'm honest I have had a real trouble with D20 due to the horrible in my eyes 'feats' system. Mainly as I don't agree with giving Humans more just to attract players to them. If folks play an Elf for the rules, change the players mind not the rules to try and make him pick what many see as a weaker race. I was just of the opinion they should have made classes that would make folks rather play Humans, or trust in players to play what they wanted, rather than jangle rules in front of them as a reason to play a Human.
Although it does occur to me I should probably just pick up D20 again to try one more time, if feats still annoy me I can always Doctor the rules.
The other reason to look at D20 is I am a fan of Forgotten Realms, and there are quite a few D20 lines out now that look quite interesting.