Best fictional evil genii

Hannibal Lecter: Silence of the Lambs by Thomas Harris.

Brain: Pinky and the Brain

Professor Moriarty: Sherlock Holmes by Sir Arthur Conan Doyle.

Doctor Moreau: The Island of Doctor Moreau by H G Wells.

Rotwang: Metropolis

Lex Luther: Superman

The Joker: Batman

I'd like to add Dr Jekyll and Mr Hyde but he wasn't an evil genius in either form. He was more one or the other, depending, and was more a character built of shades of grey rather than black and white.

The art to being an evil genius is to mash the face to express smugness, hatred, violence, arrogance, determination, secretiveness and laughter all at once. And don't forget to glare with widened eyes to show your insanity. It takes a while but you can do. Once you do you're all set to scare the wits out of any kid you like. Unless you actually are a genius or evil it probably won't work on adults and you'll just look silly.