Easiest: Dayton (DAY), Reno (RNO), Destin (VPS) and Huntsville (HSV). Connected right to the airport. Walk across the road to get your car after grabbing your bags. And usually very nice counter folks.

Not too bad: St Louis (individual rental locations vs. rental center) however shuttles can be slow sometimes. Honolulu (HNL) - it is connected to the airport, however lines can be really long sometimes. Denver (DEN) is also not too bad. As is Orlando (MCO) - unless you do an offsite which can be tricky. However I did recently rent at Routes at MCO over Thanksgiving. 4-door Jeep Wrangler for only $100 for the week. Clean car, easy to rent from, nice people. I recommend them. Ft Lauderdale (FFL) is also not bad. Connected to the airport, but can suffer from long-lines just due to airport volume.

Key West should be easier than it is, but is kind of tricky. Yes most of the major counters are in the airport across from baggage, but for some reason most have computer issues that make them SLOW. The counter folks are nice, but even 2-3 people in line can take 30-60 minutes or more. And to my knowledge you can't bypass the line with status.

Horrible / Do Not Recommend: Las Vegas (LAS). Lines to get on the bus can be 30 minutes or more sometimes (usually when it is over 100 out). LAX. BWI. I'm sure there are others I am missing.