As far as weather goes, having spent a few years in awesome San Diego, I can say that is is definitely beautiful pretty much every day of the year (except that two week period in January when it rains constantly). Having said that, it soon became very, very boring. Plus, Christmas in California just isnt Christmas. Surfing before Christmas dinner just didn't seem right.
For fun, again having lived on both coasts, I'd have to go Northeast, just because everything is encapsulated within those big cities. For example, If you're in New York and its 2am, you're 3 minutes from anything you could imagine that you'd want to do. Plus, you don't have to hop on the freeway to do it.
For me, personally, my passion is politics. I love the strategy and intrigue that go along with it. I'm going to law school because I want to get into politics as an elected person. Therefore, for me, the most fun would be to go to law school in a state capital or Washington. It is why I am applying to Albany, Maine, Suffolk and IU-Indianapolis and Catholic. They're the best schools, in my areas of choice, that my numbers might get me into (3.71/152).
So, to answer the general question, the concept of "most fun" I would think would depend upon your interests. If you are a club hopper, then obviously LA, NY, Chicago or any other big city is your best bet. If you like hiking and other outdoorsy type stuff, then any New England School would do (also works if you like to ski). If sports is your thing, Chicago, NY, Boston, Philly would probably be for you.
Thanks for allowing me to ramble. I have to get a handle on the whole "conciseness" thing.