Dollar, Avis, Budget and L&M. They are all a short walking distance from luggage pickup and very convenient. The only one I don't recommend is L&M. IMHO, stay with a well know rental agency. If you do a search here at the DIS boards about 50 percent have had good experiences with L&M and the other 50 have not. Not good odds, we are one of the 50 who have not. Dollar has some good prices right now at MCO. We have used Avis, Dollar and National many times at MCO with good experiences each time.
We use Hertz and even though they are off property we are driving away in our car in 5 mins. When we go to leave we drop the luggage of at curbside, return the car. I have used National and find Hertz much easier and just as fast.