If you could instantly become an expert in any skill or field, no matter how random, what would you choose and why?

This question made me realize that I could become an expert in anything I wanted at any time; there is nothing stopping me. I guess I'm just not interested.

Survivalist knowledge would be practical in emergency situations. You'd get things like first aid, nutrition, self-defense... Seems pretty broadly applicable.

e: Thought of something. I want to be an expert assassin. Really top grade action movie shit so I can kill whoever I want while maintaining safety, secrecy, and connections.