Agreed on any of the onsite airport companies - most of them are all owned by one of three companies.
Enterprise owns Enterprise, Alamo and National.
Budget owns Budget, Avis, and Payless.
Hertz owns Hertz, Dollar and Thrifty.
So, 9 of the 10 onsite locations are owned by the biggest rental companies on the planet, and there is Sixt who isn't a small operation either (I've also rented with Sixt in the past when they were offsite).
They won't be the cheapest, but they'll be the most "reliable". The budget brands tend to be Alamo, Payless, Dollar and Thifty.