My favourite pie is lemon meringue, but it's not really a seasonal pie, rather an any time of year pie.
For summer pies, I wouldn't turn down a piece of any of these: cherry, strawberry/rhubarb, peach, and blueberry. Homemade though. None of the store bought, or frozen ones, compare to homemade.
Fall favourite is apple. I do, though, usually have a small piece of pumpkin pie at Thanksgiving.