Whomever gives you the best rate. I only rent from the major companies (most recently Budget). I use carrentalsavers dot com to find the best deal. You usually don't see good deals/price drops until 2 weeks before a trip. My most recent trip, prices didn't really drop ($127 with tax for minivan week) until under a week before. You can book and cancel as often as you like (no penalty).
Also check the Transportation board because discount codes are listed there. I used carrentalmomma.com and got a good rate coupled with an Alamo Insider discount. Alamo/National is at the Dolphin if you want to do ME from the Airport. We have done rentals from the airport and also ME from/to the airport and rental car from Dolphin the day after arrival until the morning of departure.
Like everyone else is stating whomever is at the airport and who's got the cheapest rate
Check sites like car rental savers and also check the Transportation board to discount codes
Good Luck!
I'm used Thifty recently and it was fine although I almost missed the entrance for them when returning the car. If you use thrifty and are I interested I have a one car class upgrade coupon I would be happy to send you or anyone. And no I don't work for Thifty. My travel agent sent them to me and I so I have two extras.