That’s tough.
D&D 3.5 holds a lot of close memories that certainly rank it rather high; with characters that were a lot of fun and worlds that have been silly and enchanting; but I’m not sure if favorite is a good word.
Don’t Rest Your Head was a brief affair that ended in tragedy but I’m still left missing the concept (though the implementation is meh)
Fate would get more points if the Aspect system didn’t fill me with so much anxiety. That’s a shame though because I seem to hoard Dresden Style Fate characters (whoops)
I suppose that leaves my current mini-obsession of Dungeon World!
DW’s bonds might give me similar uncomfortable feels to FATE’s aspect system, but a guiding hand from a patient GM has done wonders there. On top of that the system is incredibly simple and the world gets built while you play! How exciting!
DW and FATE might actually tie here. They both include a lot of things I really love and have left me with brilliant experiences. They might not ever hold the love in my heart that Exalted does, but they come pretty darn close.
Of course, there are still plenty of systems I haven’t had the opportunity to play (though someone (or should I say somewhere) is trying to remedy that); as well as several I have played that don’t quite make the cut (Sorry Paranoia, I will never love you).