IMHO, the best compromise between aesthetics, ride, performance and cost is 18".
19s and 20s hurt performance and ride while costing a lot more to shoe them, for me, that's not offset by a better aesthetic. In fact, once you get to 20s, you're approaching the Donk range for these cars.
I'm sure there are others who disagree and will chime in with their opinions. That's OK and why America, and this forum, are so great!