This discussion could go on forever.

There was a guy at Expo with 18" wheels, and he seemed to do OK.

The road racers are going to prefer 15", with a few opting for a combination of 16" wheels/tyres if they think they have the right combination of rubber, weight, and weight concentration.

Some are forced to use 17" because of series rules, but you shouldn't take this as evidence that they'd prefer it if left to their own devices.

Who knows...maybe one day a specific tyre's availability in 18" will move us all in that direction!

In the mean time, if it's looks you're concerned with, it's in the eye of the beholder, and you don't necessarily want us telling you what you like.