If you're going in the spring or fall, definitely San Francisco. In the summer, Chicago.

I was going to vote for New York, but I see that you've already been there!

I would say, based on the places that you've already visited, either San Diego or San Francisco. SD will have the beaches and the water sports, but SF will have more cultural activities and fine dining. It's also close to the Napa wineries for a nice picnic and day trip.