Wow, I guess I'll go against the grain here and say Los Angeles! The beaches rule, the women are hot, the shopping is fun, and I just love big cities, and you don't get much bigger than LA (even though SF is huge, too!). I just like the atmosphere and the climate of LA. San Francisco is also very nice, but I havn't spent as much time there. I like all of the hills and stuff around downtown SF. All of the huge bridges are nice and everything, make for some great views. In LA, you are also close enough to San Diego, which is also a very nice town. In SF, you have wine country and if you want to go for a little bit longer of a drive you have northern california which is also very nice. (Sacramento is an underrated city!)
In case you haven't been convinced already, San Francisco is much, much nicer. In addition to all of the natural beauty, it's just plain easier to see, as public transportation will take you to just about everything around the area, with the possible exception of Marin County and the Napa Valley. LA's definitely worth seeing at some point too, but you probably need a car to get around. I'd certainly rate it behind SF, even though some parts of it are cool.