Real like, I quite like spring. In-game, I can't say, since I've never experienced all the seasons in an Animal Crossing game. Winter's the one I'm most interested in at the moment, since it seems to have a lot of cool (pun not intended) events.
In real life, I love fall the most. In AC... hmm... Maybe winter? AC is more of a cold weather game for me, because I feel guilty playing when it's so nice outside.
Mm, I don't think I can effectively answer this for a while. I've only played summer so far.
In RL I like autumn best. Early autumn: it's the time of year where everything I'm allergic to starts to die... and it's still nice enough to go out without a heavy jacket.