FireBat257
The Burning Bat of Flames.
Most likely, "Egg Rock Zone 3," it doesn't really get hard when you know how to beat him. I mastered it. So, there wasn't really a hardest boss I had. Plus, Sonic and the Secret Rings, of course, the last boss on that game was the hardest, until I mastered him also. So lets just say "Last Boss On Sonic and the Secret Rings" was my hardest boss in the past.