I would have to say either Southern Cali or the Big Island Hawaii. I have lived and visited just about everywhere in the country. I like Southern Cali because there is just some much going on there. Between the different cities, beaches, and mountains there is an endless list of activities to do.
As for Hawaii I just really like it. Laid back with lots of water activities. But depending on what part of the island you live on it can make Seattle look like the desert.