Alamo.
Dollar and Thrifty are horrid in my opinion, there is a reason most business travelers avoid them like the plague. Typically (although there are definately exceptions), their vehicles are of lower quality (base models with few options), higher mileage and of the rental agencies are some of the more aggressive with pushing add-ons (insurance, etc.). Enterprise would be next on the list, but for airport locations, much better than their local offices (and they share fleets often in large locations with National/Alamo - they are all part of the same company).
Sixt is new to the states, so their reputation here has not been earned really yet, their vehicles are mostly going to be newer (just by default of building their fleet), but how they maintain and replace their cars is going to the be big tell. That said, they are huge in Europe (think Hertz of Europe).