The answer to this question is very subjective. It depends what sort of lifestyle you want to lead. Personally I'm not a fan of the Haight. It's foggy and has a lot of grungy people hanging out. SOMA has less of a neighborhood feel but the weather is a lot better, you're walking distance from tons of restaurants and bars as well as many different neighborhoods. North Beach is nice but you have a lot of bridge and tunnel crowd there and getting anywhere south from there is time consuming and annoying.
I would spend some time in all these neighborhoods and decide. The mission is also a great place. Good weather, centrally located, and lots of bars and restaurants... more