Overall, most of the major rental car agencies we have used have been good. Like other DISers, I would advise you to choose a company at the airport if you can.
Avis is a pretty good company, as the service was friendly and efficient and the Ford Focus we rented was in excellent shape.
Alamo is another solid choice, as you can use the electronic kiosks to make the process of getting your car quicker and if you have to, their counter service is friendly and efficient as well. The Chevrolet HHR we rented was in good shape, drove well, and was clean. In addition, they had a large fleet. National, which is owned by the same parent as Alamo, was excellent as well with efficient kiosks or the counter (we used the kiosks) and clean, well maintained cars, like the Chevrolet Impala we rented. I would use caution if renting with Thrifty, as our agent Iris, while not unpleasant, was very upsell happy (more so than other companies) as she tried to sell us a larger or sportier car (Mustang) and pressured my mom into purchasing a $34 Sunpass transponder, which we only used $4 of to go to/from MCO. Despite this issue, the Kia Soul we rented was a great car that was clean and well maintained and returning it was fine

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Enterprise, with excellent service and well maintained cars is another fine choice. While I have no experience with the MCO branch, despite being off airport, I would also consider Sixt, a German company now making inroads in the U.S., as they have low rates. More importantly, based on experience with Sixt's Palm Beach International Airport branch, we found the service to be efficient and friendly and the Ford Escape we rented was well maintained, clean, and had only 350 miles on the odometer

To sum it up, I would use Avis, Alamo, National, Enterprise, and Sixt but exercise caution if using Thrifty. Also, I have used Hertz, but not in a while and not at MCO as they are usually more $, but they were good too.