Honestly, I've been searching for a "favorite" for a while. I'm a fan of GURPS and Chaosium's Basic Roleplaying (BRP). I have fond memories of The Fantasy Trip (now out of print), and my bookshelf is full of indie systems I'd like to try at least once because they look cool.
Right now, my current favorite is Spirit of the Century, an evolution of FATE 2.0 and a prototype for FATE 3.0. The system uses only a standard list of skills to cover every possible action (conceivably), a standard list of Stunts for unusual abilities, and Aspects which are arbitrary phrases describing what makes the character truly different. A player can "invoke" one of his Aspects and spend a Fate Point to alter a skill die roll; he can also "tag" Aspects of other characters, or the situation itself, to give himself a bonus. Occasionally the GM can offer a Fate point to "compel" an aspect, which prevents a player from taking a course of action contrary to the Aspect (e.g. a character "Raised in the Jungle" might not know how telephones work.).
The Aspect system subsumes advantages, disadvantages, relationships, backgrounds, and skill adjustments into one elegant system, and rewards creative use of story elements. (The damage system even involves adding a type of Aspect called "consequences", which NPCs can tag or the GM can compel: "Broken Leg", "Blood Loss", "Punch Drunk" ...) I wasn't too keen on the FATE 2.0 version, but the SotC version of Aspects really make the game interesting, especially as the evening wears on and players' Fate Points run low.
SotC is partly Open Content; the SRD is at http://www.crackmonkey.org/~nick/loy...te3/fate3.html (and a few other places).
"On two occasions I have been asked [by members of Parliament], 'Pray, Mr. Babbage, if you put into the machine wrong figures, will the right answers come out?' I am not able rightly to apprehend the kind of confusion of ideas that could provoke such a question."
- Charles Babbage (1791 - 1871)