All bosses from Sonic the Hedgehog 4: Episode 1. I realize this game is incredibly lazy with its bosses butt fuck it. Final boss is actually pretty neato.

Timestamps:

Dr. Robotnik (Splash Hill Zone) - 0:03
Dr. Robotnik (Casino Street Zone) - 0:57
Dr. Robotnik (Lost Labyrinth Zone) - 1:54
Dr. Robotnik (Mad Gear Zone) - 3:33
Dr. Robotnik (E.G.G. Station Zone) - 5:30