When I went to university, the place was full with plane trees. The leaves - in that beautiful shape and color - cover all campus. Besides the charm, the weather is not too cold or too hot. It's perfect! So, the answer is autumn.
The leaves are a nice memory, although I just realized that I don't have any photos of that period. Can't believe that I miss that!
I've got a lot of random pics of the city that I'm living in, so I'll use this as an example for that perfect atmosphere :P
Thanks to @rambo and @missy for the homework!
duitall:
I flourish in warm weather!