Whichever item makes the most improvement to the particular car for a specific track... Say you're driving a POS car, makes very little hp, handles like a boat, weighs a lot, and has crappy tires...worst case scenario. What will improve it the most? On a high speed track a turbo might improve your times the most. On a twisty track, probably tires.
I think a better question is what would be the best improvement per credit. But even still I think it really depends on what car you're starting with, and what track you plan on using it.