Autumn is by far my favorite time to camp. Summers are ok if you have lots of trees for shade. Camping in a pine forest is amazing because a layer of pine straw adds so much cushion to the ground (but not great for the rain fly as sap drips on it).

I've never done a true winter camp, the coldest I've camped in is -5 C / 23 F and that was just the low temp overnight.

Late spring would be my second choice for camping after the ground has defrosted and it's not so muddy.