That depends on the individual. Beginner, advanced, cold water, warm water, etc. Most will say Hawaii. Having just returned from there it would say it was the best surfing I've ever done but it was very crowded and you have to be a little aggressive if you want a wave at a premium spot. Despite that, there is good surfing on all borders of the mainland to include the Great Lakes and Texas! Best thing to do is go to local surf shop and ask about the more popular breaks.
Hanalei, HI most of the year. I also do love trestles San Clemente California. But for my number one spot in teh USA for surfing for me is Sebastian Inlet Florida my home town (ok now everybody please rip me apart).