Let's list off the actually good Sonic games. And I don't mean "good for their time", I mean that without nostalgia goggles, without ever having played them before, you could play them today and have an enjoyable experience playing them.
Well, obviously, the first 3 Sonic games, as well as Sonic CD, Sonic & Knuckles and maybe Knuckles' Chaotix. Those still hold up quite well, in the same way that you could easily pop in Super Mario Bros. 3 and have just as much fun as you did 25 years ago.
And then we come to quite a gap. Sonic 3D Blast is mediocre at best. Sonic Adventure is janky as all hell, don't tell me it's not. People bitched at Arin (Egoraptor) for shitting on that game in their playthrough, but many things he said were absolutely true. Adventure 2 is a bit better, but not by much. Sonic Heroes and Sonic Rush are so forgettable I barely remember they exist, to the point where I had to look at the Wikipedia page to even remember what Sonic Rush's name actually was.
Sonic 06 is an abomination and deserves to go down in history as one of the worst games ever made, and really Sonic Boom isn't much of an improvement.
The Secret Rings and the Black Knight are both shit, as well as Sonic Unleashed. If Sonic Unleashed was just running stages, it would probably be better, the problem is the God of War-lite stages and walking around they put in the game, because I don't know about you, but when I want to play a Sonic game, I have to make sure I'm strolling around town talking to people every 10 god damn minutes. They clearly didn't learn their lesson from 06.
Sonic Colors was surprisingly good and so was Sonic Generations. Meanwhile, Sonic Lost World is basically "we can do Mario Galaxy too" but not good, and Sonic 4 was an attempt to go back to basics that failed miserably and sucks pretty hard.
So in over 20 years of history, we have Sonic, Sonic 2, Sonic 3, Sonic & Knuckles, Sonic CD, Knuckles' Chaotix, Sonic Colors, Sonic Generations, and now Sonic Mania.