I'm gonna be original and say Sonic Colors. You can talk all you want about how 06 broke the fandom, but everyone agrees it wasn't good. Even apologists will say "The game was flawed."
Colors (especially story-wise) is when you really start seeing people with radically opposing views. I'm not as butt-hurt as I used too, but I still think the game's story wasn't good, even if Ian Flynn likes it.
I mean, that's what "controversial" means. When people have drastically different opinions, not just "Classic shouldn't return", which most people seem to agree on.
I agree about Classic Sonic. He was fine in Sonic Generations because there was a need for him in that storyline. However, afterwards it seems like SEGA had no idea what to do with Classic Sonic and they just ending up shoving him into games that he had no business being in.