Ace used to win all kinds of JD Power awards and things, which surprised me as they were tiny. Then they got vacuumed up by Avis Budget Group and the last I heard, ABG was phasing out the Ace brand and rebranding them Payless. (Apparently ABG leadership wasn't aware that the Ace brand had positive connotations and the Payless brand had negative ones...)

Probably the best at service recovery would be Enterprise. Their operation, especially "home city" (neighborhood) branches, sucks, but they'll literally do anything to make you happy, because their entire management bonus and promotion structure is based on the "ESQi," which is a score determined by the percentage of people who respond "Yes, I was completely satisfied" to random post-rental surveys. Simply utter the words "I was not completely satisfied" to any Enterprise employee and watch fear and terror be stricken into their heart. Even the lowliest "Management Trainee" is authorized to do whatever it takes to remove the word "not" from that statement. Maybe that means a day free. Maybe two. Maybe your whole rental. Up to you to see what you can push for. Just don't expect Enterprise to actually take your feedback to heart and change their operation--it's far too wildly profitable to oversell themselves, make customers wait for cars to return and get washed, force people to accept an upgrade to a pick-up truck, fall for the con of buying insurance, etc. But when you call them on it, they'll fall all over you to make you happy with them, which can work to your advantage. Thing is, you have to deal with someone at the branch to get this kind of service. Their post-rental customer service is kinda lacking. IME, their call center reps are very by-the-book and won't make customer-forward exceptions to policies. If you have a legitimate gripe (like your reservation's rate/car size wasn't honored or whatever), they're efficient about correcting it, but if it's a he-said-she-said thing, like you felt forced to buy an upgrade and are now regretting it, too bad.

National and Alamo use the same crappy post-rental customer service process as Enterprise, but they lack the on-the-ground employee focus on ESQi, so your chances of complaining your way to a cheap rental are far lower. However, Alamo and National seem to run their operations a little better than Enterprise (it helps they're mostly larger operations at airports, rather than strapped-for-cars city branches), so the need for good service recovery is lessened, because the service itself is more reliable.

Hertz's operation is pretty mediocre, but when I have needed to call their customer service, I've been treated better than I have with Enterprise/Alamo/National. They're less efficient (Enterprise/National/Alamo typically replies in less than 24 hours, whereas Hertz takes a couple weeks or more) but seem to be more willing to just give you your money back. Dollar/Thrifty is pretty well integrated into Hertz's systems these days so the experience is similar.

Avis and Budget have relatively fast response times (a couple days) but sometimes are even more hardnosed than Enterprise/Alamo/National. Even getting Avis and Budget to fix a mistake that's clearly their fault (like the rental counter not honoring a coupon that the website fully confirmed as valid and applied to the rental) is like pulling teeth.

Payless...just no. Despite being owned by Avis Budget Group, they basically have no service recovery at all. Their Mexico-based call center is one of the most useless call centers I've ever dealt with. (And I hope it's not a sign of things to come, but the last time I called Budget's CS line, I think it was the same Mexico-based call center...)

That leaves, what, Sixt, Fox, Advantage, and E-Z plus a smattering of regional/local players?

I've not had fantastic luck with Sixt's customer service, but I've also not had much reason to call them. Their CS strikes me as very German: efficient at telling you, "No."

I've never had a need to call Fox CS because my one rental with Fox was enough to make me think twice about renting with them again...

Advantage and E-Z is a mixed bag. They seem to be trying harder than Fox (and waaaay harder than Payless) but just don't quite reach the level they're aiming for. I think they intend to provide a good product with good customer service, but their lack of name recognition and the correspondingly low rates they are able to command prevents them from having the resources to put out a good experience for customers. That said, I think if you had an issue and contacted them, they'd probably at least try to work with you a bit.

And that's my summary of the rental industry based on 15 years in it on both sides of the counter.