Shadow does not exist. He is not Knuckles, Sonic, or Tails. Or Dr. Robotnik and NO HIS NAME IS NOT EGGMAN YOU FUCKING MORON I WILL SLIT YOUR THROAT AND YES THAT IS A WRITTEN THREAT. Metalsonic (Sonic 2 version), because he was hard as hell to beat. No rings and it's hard to figure out exactly when to jump while he's spindashing. Hardest boss IMHO, Robotnik was easy compared to metalsonic.