Technically GGXX. Personally SFIII:TS.
GGXX is better rounded, better balanced and offers some of the most variety of any 2D fighter in existance. SFIII:TS just feels better to me and is something I adore, but the whole underlying concept of constantly trying to trap your opponent into a combo takes away from it in my eyes as a freeform battle. So my vote is for one of them, I'm not sure which.
Note: The comment on SFIII has the emphasis on "trap," I'm well aware it's benefical in any fighting game to lure your opponent into a combo whenever possible. Unless you're playing as Chipp where easy combos and a super do less damage then tapping HS.
