Heat, although I don't like it as much as when I was young. In the middle of the summer, you can always get outside in the mornings and do stuff before the heat really kicks in. I often go for a 40-60 mile bike ride and am done by 11. You can't do that in the winter.
I also lived in a ski town in Colorado for a couple of winters, and cold there is not an issue, because you can still get outside and do something almost every day of the year....Most nights the temp would dip into the single digits or teens, sometimes well below zero, but the sun would often shine during the days.
The problem is when it's cold and dark and you are stuck inside all day with nothing to do outdoors. I never understood why the Chicago Bears, and other pro teams,were so popular until I lived in Chicago. There's literally nothing else to do during the fall on a weekend. The two goes crazy from Memorial day to Labor day and then the vibe completely changes.
You can still bike in the winter - you just need the proper gear. My brother rides all winter in Minnesota. And a quality thermos keeps his coffee hot even after a couple hours of biking.
And are you serious about there being "literally nothing else to do during the fall" in Chicago besides go to Bears games?