[Boudica]

In the UK: Winter, because my particular house gets so hot I can't actually sleep at night, and I HATE being too hot. At least in the Winter I can warm up. However, those I live with have a very frustrating habit of leaving the heating on too high, for too long, and it's the same problem with not being able to sleep all over again.
I also suffer extremely strong allergies from about May until November. It's genuinely like having a stinking cold that whole period. Often, allergy flare-ups result in my immune system being compromised and I actually do get sick with a nasty chest infection about twice a year.

In the US: Couldn't care less. I love all seasons as there is AC and plenty of sun. I have no pollen/seasonal allergies as the grass/local flora are different species to those in the UK.

tl;dr - Winter because I am literally allergic to England from Spring to Autumn.