The maximum choosable is two.
Notice I've included the Lock-on games from S&K.

I'd have to say it's Sonic 3 and Knuckles, because of the Super Emeralds, then Sonic 3D Blast, for no real reason.

Of course you know what I chose. STH 2 introduced Super Sonic. 'NUFF said.

For the extra length, the super emeralds, and for introducing knuckles (even if they have shot his character all to hell now) S3K!