All Sonic related, surprised?
Well, there's a level in Sonic Lost World that makes me regret the day I bought it, it's Lava Mountain Zone 2. The entire level is garbage, you're stuck on grind rails and are constantly surrounded by instant kill bombs, it's torture. I finally beat it, after about 200 lives, lol.
As for bosses, Dark Gaia in Sonic Unleashed. You don't fight him as Sonic, but as Gaia Colossus. It's pretty bad, lol.