Tires followed by disconnecting exhaust pipe servo.
So much one can do - so little money ))
Tires is also a very good one. I'm cheap so I'll just burn through my stock first while I get used to the bike, then get the set I want. If you are buying a used bike, obviously tread wear will bump this to #1 if it is bad enough, or if the tires are shitty enough. Point here is to be as safe and confident on the bike while "dialing" it in.
That said, chain and sprockets would be my next choice, followed by levers.
So in no particular order for the top 3 at this point would be:
1) Tuning suspension for your body weight, and riding style, environment
2) Tires (same as #1 - riding style, environment)
3) Chain/Sprockets (same as #1 - riding style)
3) Chain Adjusters
4) Steel braided brake lines
5) Fender eliminator
6) Turn signals
7) Wind screen
Keep the mods coming, what else do you like to swap out from OEM?