I've played a lot of systems, mainstream, obsure, and hombrewed. My favorites change from time to time.
Right now I like Unisystem, all forms,whether its Buffy/Angel, All Flesh Must Be Eaten, Armageddon, Terra Primate, or Witchcraft. I like the simplicity of the rules, and the character creation system, and I am a huge fan of BtVS/Angel.
The system I guess I've like best over the course of the years is Champions. It allows for almost anything as far as character creation goes, and while I have loved the superhero verses I've played in, I've found it lends itself to just about any genre.