all seasons in chicago have their pluses and minuses, but i can loosely rank them as follows:
1. fall - my favorite time of year for riding, it's typically cooler and dryer. the lakefront path also gets less an less crowded as the season goes on.
2. spring - often nice riding temps, but rain and lingering winter nonsense can make it awfully messy. lakefront path starts to get more use, but usually still tolerable.
3. winter - the lakefront path is blessedly vacant, but strong winter winds along the lakefront can be utterly beastly at times. winter mess is also a minus.
4. summer - my least favorite time of year to ride. it's usually too warm and humid for my tastes, and the lakefront path becomes a completely unrideable clusterf@*! in the afternoons.
however, in several weeks i'm moving to a new condo that will shorten my one-way commute distance from 14.5 miles down to 4.5 miles, and will also eliminate the lakefront path entirely from my route, so my bias against summer riding may change a bit.