By far the easiest is National followed by
Alamo.
Whichever company you are going to use, join there free loyalty program first to speed your wait at the counter or even by-pass the counter all together and just go straight to the garage and pickup the car.
National has Emerald Club, book a mid size and pick anything in the Emerald Aisle (guaranteed mid size and up). Go straight out to the garage, pick the car you want from the Aisle that you booked and drive it to the exit gate (keys are in the vehicles). Hand the clerk at the exit your drivers licence and credit card and your on your way. If you need a specific vehicle, say a minivan, you can't count on there being one in the Emerald aisle (although quite frequently there is). In this case, book the vehicle class that you need and you will pick from that aisle (ie minivan)
Alamo Insiders club is similar except that you book a size class of vehicle. Most convenient way is to use the "online check-in", use this the day before you leave home, print the paperwork and take it with you, when you arrive in MCO you would proceed to the parking garage, follow the Alamo signs and then find the row or cars for the size class that you booked (say, full size), pick something from this aisle that you like , load up and drive it to the exit gate. Hand the attendant the paperwork (from your online check-in), your drivers licence and your credit card. You will be checked out and on your way in no time.
Budget uses a program called Fastbreak and Rapid Rez to help you bypass the counter and get your vacation started sooner. Go out to the kiosk in the garage and there you will find the vehicle that has been assigned to you (sorry you don't get to pick your own vehicle, but if you really object to the vehicle they have assigned you, see a lot attendant about a change)
Avis is Budgets parent company and their program is called Avis Preferred Service, works the same way the Budget program does (see above)
Hertz has there Gold Plus Rewards Program. Similar to Budget above, there is a Gold Rewards counter out in the parking garage and they will give you the paperwork and tell you which vehicle you have been assigned. Hertz has a car class called "managers special" which can be a cheaper alternative, you are assigned a vehicle from any class (compact or larger), the last 2 times we used this car class we were given a mid-size SUV, but it's definitely a YMMV category. They have a nice email or text feature that you can opt into where they will send you the info which car you have been assigned. This usually comes an hour or so before your scheduled pickup time so as soon as I turn my phone back on when we arrive at the airport, I get the message about my rental vehicle.
Dollar and Thrifty are owned by the same company and have similar programs that let you by-pass the rental counter and head out to the garage.
Dollars program is called Dollar Express. A car will be assigned to you (don't get to pick). They have a size class called "lock Low and Go" which allows you to pay a Compact price and they will assign you a vehicle that is Compact or larger. (Similar to Hertz Manger Special program above)
Thrifty's program is called the Blue Chip program and works the same as Dollar Express, skip the counter and car assigned to you in the garage. They have the Wild Card program, pay Compact price get a Compact or larger.
With both of these programs (wild card and lock low n go) the vehicle assigned seems to be all over the board with most people not actually getting Compact but something larger from Full and Mid size cars to small/medium SUVs and even the odd minivan depending on the time of year. The program allows the rental company to utilize some of it's slower moving inventory (basically if it's not moving, this gives them an option to make some money on these vehicles).
There are several other companies in the rental car market, some on site and some off-site requiring a shuttle. The above are my picks for ease of use at MCO and most major airport locations in the USA.
Prices can vary greatly and change often so book what you can afford with a company that you feel comfortable with and then check back often to see if the price has dropped. If prices drop, book a new rental with the lower price then go back and cancel the old one. Rinse and repeat until your vacation starts as prices can drop right up until your rental date.
Don't book a pre-paid reservation unless you find an awesome deal or it's the best price available the day you are scheduled to leave. Pre-paid rezies are charged a change fee ranging from $15-$50 if you need to change the rezzie so you want to be sure that this is the price that you are comfortable with and your flight times etc are nailed down.
