Here's my take on Sonic 2 since I have a few minutes:
I agree with everyone else that says 3&K is the best entry on the system. But that's not what made it to the finals.
Sonic 2 is a platformer that does so many things right. It has areas that require speed and precision. It has areas that require complex platforming, but not overly so. The game rewards speed running by offering time bonuses at the end of each act that add up to extra lives. The game also rewards slow exploration by hiding power ups, rings that add up to extra lives and special stage access, and extra lives themselves across each act. This is excellent game design in my opinion.
The presentation is outstanding. The zones across the game are all colorful, unique, fun to look at, and have incredible music. Some of the best music of the 16 bit era is found here. Sonic 2 is a game that is remarkably fun just on how good it looks, sounds, and feels.
We've all played Sonic. I assume we all know how the physics work. I personally think they work great. The spin dash nullifies some of the more frustrating parts of the original game where the physics worked against it, like going up a steep hill or ramp from a stopping position.
Where Sonic 2 has a weakness is the special stage. I do not enjoy the half pipe stages of Sonic 2. It's too difficult to react to and some of the later ones can only be beaten with attrition and memorization. And the idea to start at 0 rings was not a good one, making you recollect or search out 50 more before you can have a chance at the next emerald. It feels like work. Personally, when I play Sonic 2, I just skip the special stages. Its more fun that way.
I admittedly have not spent as much time with Gunstar Heroes. It is indeed an excellent run and gun. The music is great, the weapons are inventive, and bosses are usually pretty fun and intense. But that board game stage... if I have to critique Sonic 2 for at times feeling like work to unlock the best ending, I can't forgive Gunstar Heroes for having an entire stage required to beat the game that feels like a long chore. It doesn't make the game bad, far from it. It is, however, a flaw that has kept me from returning to it countless times like I have for Sonic 2. If I had to make the desert island choice between Sonic 2 and Gunstar Heroes, choosing Sonic 2 is a no brainer.