The only bad games in recent years were Boom and Lost World 3DS, but both of those were made by outside developers. I don't think Sonic Team has been bad for a very long time, and I'm eager to see their next game, hopefully for the 25th anniversary next year. That doesn't necessarily excuse the quality of the other games... I'm just saying that the main-series games are most likely safe.
(Yes, I actually enjoyed Lost World for Wii U a fair amount. I'll agree that it was a step down from the formula established in Unleashed, Colors, and Generations, but once I learned the new controls, I enjoyed it. It wasn't mind-blowing or anything, and it had its flaws, but I don't see how people could honestly call it a terrible game. Disappointing, perhaps, but certainly not the "return of the dark age".)