Depends, are you racing expert? That's where every little thing counts when everyone is basically running the same lap times.
I really like the orion r10.1 pro and the Trinity 24k motors right now. No on can beat me in a drag race down the straight and I have plenty of power coming out of turns, tons of torque.
My other buddy swears by his hobbywing ESC though and a schurspeed custom built motor. Batteries also figure in as well and most of the fast guys where I'm at are using orcas not orion or anything else. So many things you can do for stock buggy. Cutting weight on rotational mass etc etc..