I've rented from several agencies at MCO in the last 10 years - Alamo, National, and Thrifty.
I truly rate the service at all three equal - very good to excellent. Once in a while you will come across a rental agent who is bored or apathetic, but I've never had one be rude to me, ignore me, or be dismissive of me. I used to get the hard sell for additional insurance as well, but I have since joined the frequent renters clubs of all the major agencies, which eliminated the saes pitch and speeds the rental process up immeasurably (shorter lines, no paperwork, etc.)
It is important to note that I am always polite, friendly, and happy when I deal with the agents, without being pushy, condescending, or dismissive, myself. I often chat up the garage agents at the car return; they're a pretty good bunch of folks.
I also rate all three companies equal when it comes to quality and cleanliness of vehicles. All of the vehicles I've gotten at MCO, from all three companies, have always been in perfect working order, and absolutely clean. Most of them have had low milage, too; I believe that the agencies at MCO replace their cars fairly often, so it's rare to get one with more than about 20,000 miles on it. Most of the rentals I've gotten in the last 10 years have had less than 5,000 miles on them, and several have been brand-new - less than 1,000 miles.
I will absolutely rent from all three agencies again in the future with no hesitation.
I have rented from Dollar once. That was way back in 2002, so my experience is not recent, but I found Dollar to be as good as Alamo, Thrifty, or National, in both service and vehicle quality.
Note on prices - Thrifty often has the lowest at MCO, followed by Alamo. Dollar sometimes runs sales that are competitive with Alamo. National is in the middle of the pack, so I rarely rent from National, but I do enjoy the Emerald Isle service, so if the price is right, I use it. When I book, I typically use coupons and discount codes from my Sam's Club membership (BJs and Costco have similar coupons), and I play with them to find the cheapest rate from any of the major companies. It's also helpful to look for discounts on MouseSavers.com, or in Entertainment books or other publications. Mixing a coupon from one source with a code from another can often lead to greate savings.