The only Sonic games I have much nostalgia for are Sonic CD and Sonic R (mainly because of the soundtracks); there are also a few fan-made Sonic games which are better than most of the recent ones (e.x. Sonic: After the Sequel, Sonic Time Twisted).
I also did like Dr. Robotnik's Mean Bean Machine (apparently it's a re-branded version of a Japanese puzzle game called Puyo Puyo which is on Steam for people who like Tetris-style games).
My opinion in retrospect is that the Sonic games were always kind of badly designed - basically the "speed" was the only real gimmick, the abundance of rings basically meant you didn't have to worry much about taking damage - and usually you could just hold down the right controller button and jump a few times and you'd easily win most of the levels. (In comparison, while the older Mario games didn't have as unique graphics or soundtracks, they required more skill).
(There was also a huge difficulty spike in some of the older games, like Sonic 2 - where you'd have to fight the final bosses without any rings).
Most of the newer, 3D Sonic games are basically just a cash-grab, and try to make Sonic more like The Legend of Zelda or other popular franchises just hoping to sell (oddly, even though they get bad reviews, they apparently still sell well). I'm not totally sure why Mario-spinoff games, such as RPG, sports, party games, etc tend to do well in reviews, while Sonic games that don't stick to the original formula suck, but that seems to be the trend.