FD is sonic's best stage. As stated before: the lack of platforms is one thing.
Another: sonic's recovery is best.
-it is possible to grab sonic when he uses UP-B, if done correctly sonic can be gimp killed much like snake when he uses his UP-B. BUT in FD, you can use UP-B into the lip and wall jump to the stage, thus preventing a gimp. and in all other situations, you can just use up-b further away, or use a different move to recover.
-platforms make it different for timing sonic's Dair and commonly cause (atleast me) to hit the landing delay.
I dont know about u guys, but whenever i play on platforms, I'm always trying to get to the ground floor, being on a platform is always a disadvantage because they can run under you and hyphen, Utilt, or jump at you and aerial... and there isnt much you can do but avoid it if you are standing on a platform. Especially with sonic, he has no moves that hit below a platform.
I find myself just ASC'ing between or under the platforms, and just staying off them... all in all they just are a big wall for me that I try to avoid... The only cool thing is that Sonic can abuse the same advantages of them (U smash, utilt, Fair, Uair)
but all in all.... much easier to play on nice, flat, solid FD.