While I'm not going to promote it as best, I'm going to second or third the opinion that upstate NY is at least very good. Right out of my door I can ride out into nearly endless rolling hills and farm country in one direction; heading vaguely north I can easily get into the lower Adirondacks either around Sacandaga Lake or the previously mentioned Lake Placid and points north for decent climbing. I dont see much in the way of traffic once I go the 3-5 miles out of town. Roads are very well kept. The problem, if there is any, is that the riding season is pretty much April through October with May through September being best. You can ride in March and November with minimal cool weather gear though. I like it that I dont have to get in the car and drive and park to start a ride.
North Carolina around the Blue Ridge Parkway was amazing. Something like Asheville maybe, if you like it there and can get a job there to support you. I dont think I could have lived there, but the cycling was good. Weather was better year round (certainly compared to NY) and the mountains were something truly special.
North of San Diego, provided you could get out of the city easily enough, seems ideal to me. The few times I have been there (unfortunately without my bike) it seemed you could find whatever sort of ride you wished and the weather is literally cycling friendly all year round. Seriously thinking about moving there if the opportunity presented itself.