I love every sonic game I've played. The best 2d one was obviously Sonic 3+Knuckles. I mean...HYPER SONIC!!! He is the most ultimate character ever made!!!
I like SA1 and 2 equally. They are good for different reasons. I like SA1 because of its variety (each character was completely different) and the adventuring involved (I like rpg's). I liked SA2 because it lasts so long! I still haven't got all the emblems yet (I'm 7 away from 180), and unlike SA1, getting all the emblems actually gives you something: the original Sonic 1 Green Hill Zone in full 3d!! I can't wait. I also can't wait for Sonic Heroes.