Talking about weather, Spring is my favorite because the weather is still cool, but it's getting comfortably warmer.
Fall is a close tie with Spring for weather, i love how crisp and refreshing the air is.
But generally speaking, Summer hands-down. Most of the time it isn't too hot in Wisconsin. It's usually only 75 to 85 F. I LOVE summer mornings and evenings. I get up early in the summer (this is no lie) because everything is so pretty and cool in the shade. And it's great how late the sun sets (sometimes 8:30pm!) -it gives you a nice long day. And by summer, most of the grass and such has recovered from the winter sand-dump. And of course, no school!