This is a hard one, I like winter....but only a proper winter. Where it's freezing with ice and snow. So we're able to iceskate. But I also love summer, because you can be outside more often without getting cold XD. And go swimming, going out and sitting outside with family and friends the entire night until it's like 1am. I think overall I like summer more.
I have to say i prefer summer, I can do so much more and it is not always raining, and if it does snow in ireland you can never get any where as we have no support to deal with lots of snow.
Summer -- Winter -- They both have there down sides and up sides.
Personally, I prefer winter; it's easier to stay warm than it is to stay cool. Just bundle up in a few blankets, dress warmly, and drink a hot beverage. Although it is true that it is easier to fall under illness in winter, you could always reduce those chances by basicly drinking more orange juice and avoid unhealthy snacks a little more often.
Sure, with summer, you have swimming, but being wet in strong sunlight just makes it easier to become sunburned. It's easier to play outside in the summer, but during the season, you basicly have to hope with every fiber of your being that your AC doesn't destroy itself from being over-worked.
For me, it seems as if there is no spring or fall. Where I live, those seasons seem to pass by in less then a month.