I first want to state that I never played the Game Gear version.
When Mania came out it was critically acclaimed because the game went back to its roots and was a best of/reimagining of the 2D Genesis/Megadrive era games.
This is the same thing but instead of being a best of/reimagining, it acts as a true sequel. Forget Sonic 4. This is what it should have been.
It feels and looks just like Sonic 3 & Knuckles. As S3&K added more "story" elements to the franchise this takes it a tad further. All the old animations are there but there are plenty of new animations that keep the game fresh. There are tonnes of new Robotniks, environmental assets that either help or are a hazard. Nack/Fang makes for a great villain and the levels are interesting.
The game isn't too hard either. While the Special stages can be a tad frustrating they're not as hard as previous games and gives you the chance to retry but at the cost of a life.
I hope to god Sega pick this up and officially release it. But even more, I hope to god for a sequel. As much as I want a true 2D Sonic game that feels the part but looks 4K and stunning art (think Rayman Origins) this is absolutely the second best thing.
For the negatives: I wouldn't really say there are any. Some things like the soundtrack hasn't blown me away. But that's something that likely has to take a few replays for it to sink in my head. I didn't have one glitch during my playthrough. The lack of extras I suppose is a negative. But there is replay value that you earn when you complete the game.
Sonic Mania may be the true "objective" best Sonic game out there but that's cheating. It's a best of. While the original 2D games hold great nostalgia for me and because of that I enjoy them more I can't help but say that in reality, this is probably an actual better game.
10/10 I can barely fault it.