Any voice impression of Robotnik is fine by me, though Jim Cummings impressions are highly discouraged. Most preferably you should try going for his OVA voice or his current voice (Mike Pollock).

Movie bio: Robotnik is the antagonist of the movie and the original creator of Hyper Metal Sonic. He has spent three years developing a new and improved Metal Sonic to destroy Sonic and his friends, and he also turns Shard against his group. Despite having the upper hand with both Metal Sonic and Shard fighting by his side, Robotnik is still easily thwarted.