dynamicduo is correct. San Francisco is where I grew up, and I’ve spent a bit of time in both LA and San Diego. Personally, I think SF is the best of all three, but it does fit my personality better than LA, and I don’t think there’s much going on in San Diego.
Here’s my reasoning. The location of SF opens up opportunities in a wide radius. You’re in relatively close distance to wine country, beaches, mountains. Not too far from Yosemite (compared to So. Cal) and close to central California (Monterey, Santa Cruz). SF is chock full of culture, great restaurants, and an incredibly diverse population. The public transportation system is extensive and you don’t need a car to get around, in fact it’s sometimes simply easier to take the bus. The city is only 49 square miles, so getting around isn’t too painful. There are definitely plenty of outdoor recreation opportunities in close distance to the city (surfing, biking, hiking, boating, kayaking). The vibe of SF is intelligent, creative, and more than tolerant of its inhabitants’ individuality, which I love. I find the temperate climate to be almost perfect and there are great coffee houses, too.
LA for me is too appearance- and material-oriented (looks, brands, houses, cars, money, etc.). The sky is orange with air pollution and smog. You have the beach, the desert, and a closer proximity to Mexico. Everything is really spread out and a car is a necessity (but traffic is utter madness). Outdoor activities would be the same as SF, with more people swimming and going to the beach because it’s warmer. An actress friend of mine just moved to LA and was really excited.. until she got there. After living there for a while, she hates it. Heh.
San Diego is the sleepier, slower-paced one of the three. Feels kind of like a beach town, but on a grander scale. Also really spread out and vehicular transportation a necessity. Lots of surfing, beach-going, that sort of thing. Very close to Mexico, lots of party animals pop over to Tijuana for the night. Sea World and the zoo are there, both of which are pretty impressive and worth seeing. Aside from that, I’m not sure what all else there is to do there. I could be wrong, but my feeling is that there’s the least culture here out of all three.