Howdy Mustang HotRod:
Is this a trick question, or what??????
The truth is there is no "one upgrade" that gives the best performance for a 200. It takes a coordinated, balanced package that includes intake, flow, compression, ignition, exhaust and cam timing in a good flowing head, power adders and a durable block that is balanced. It must all flow through a coordinated drive train with proper gearing, in a vehicle that is light, safe and solid, on tires that meet the needs of the package.
So, I guess the best upgrade for the most performance is a well thought out plan- that you can, realistically, afford.
Start by reading here, and then get to know what you have and go from there.
Adios, David