Depending how you define free time. I think JFK & EWR aren't too bad both have trains into Manhattan but it take 40-hour each way (if you aren't experienced). Both airports are notorious for delays so need to keep that in mind.
I think ORD isn't too bad as it is a 40 minute line on the BLUE to downtown.
The same length applies to SFO & SEA.
Houston and IND have good public transport to downtown but not much downtown.