Y'all already know that I like the plumber more than I like Sonic by well...a heck of a lot, so you'd think I'd defend Mario here. Say things like Sonic peaked at his third game or some other thing like that out of spite. But no. I won't. Sonic has a ton of great things. Music is top notch. A decent portion of the characters are great. I mean, sure I'd limit the cast to just Sonic, Tails, Knuckles, Robotnik (& bots), Metal Sonic, Mighty, Ray, Amy, Shadow, Chaos & Tikal, Rouge, Cream, the Chao, & Omega. But that cast is still good. Level design isn't half bad, though I do think they get too complicated to get to the goals. They pulled off their first showing of Sonic as an animated character much better than Mario did (Sonic Boom (the animated song) and Sonic SAT AM were much better than Hotel Mario and the SMBSS). And apparently Sonic did something right to gather such a large fanbase. And Sonic has almost always been great when it comes to voice acting. While I don't have the biggest attachment to the rat, I can respect him for leaving such a big impact on so many.
That said all of Sonic's gameplay has been mediocre at best since the beginning