Rental car
I don't know if you're just asking about the FBO itself, or the airport/FBO in combination. And I assume you're talking about a destination FBO, not one in your hometown.
For the FBO, I look for (in order of priority):
- RENTAL CAR AVAILABILITY and ease of access to same. Note: I'm usually on a business trip and it's important to get in/out efficiently. Having Enterprise available, but a $75 "delivery fee", or an hour to go get the car isn't good.
Best in class: McKinney Airport (TX - KTKI). Call ahead, reserve the rental car, and they will meet you on the ramp in the car. Throw your luggage in and you're off!
- manageable fuel prices but this isn't the only criteria
- ramp/overnight/tie-down or hangar fees, and whether or not the above-mentioned fees can be offset by fuel purchase
- friendliness/helpfulness
- hours of operation - are they open when I get there? when I want to depart?
- comfort of the lounge - much more important if I have friends/family with me
- availability of maintenance in case I need it