If you could do ONE upgrade...

Depends on the intended use of the car;
If you have limited driving skills your car will probably not get modified past the dragracing-level, so engine-power would be important.
If you plan to daily drive the car, steering and suspension/handling would be nice to be modernized a bit. Followed by brakes, tires and then engine power.
If you like to be able to hang with today's traffic, visit a race-circuit one in a while; steering and suspension/handling would be nice to be upgraded. Followed by brakes, tires and then engine power.
But if you are tired, or retired, and never drive over 55... A/C would probably be your best shot at improving your car...

:)