Of the ones you listed? Piedmont

The long term viability of the non wholly owned is not good. One major at a recent town hall told their employees that other than Republic, SkyWest, and ExpressJet; the only long term survivors of the shortage will be the WO's.

I'd go first to whichever has a large base in your home city. Nota small base, since as the shortage deepens, small bases will get closed as too inefficient. The QOL advantage of not commuting makes a regional airline an entirely different job than that of a commuter.

If living in base is not an option, then it does matter. All regionals are not alike, all travel benefits are not alike. Choose Endeavor first, followed by the three AA WO's (if you are young enough to anticipate going to AA, then you may want to put Endeavor after the others since all three AA WO offer no interview direct flow to AA). After that I'd probably do SkyWest, Republic and then ExpressJet.

The Travel benefits - if you're going to be a commuter - are important also. If you are at Endeavor or any of the AA WO's you come ahead of the offline non-revs and jumpseaters. At the AA WO's your travel benefits not only include your immediate family, but also include up to 13 others of our choice. Traveling non-rev is a pain, more so when trying to bring family along, and even worse when you are an offline non-rev. If you are going to commute, getting the best possible travel benefits can make life as a commuter easier..... I've lost count of how many times - back in the day - myself and other regional pilots got on our mainline flights home, while the TSA, Mesa, Republic, SkyWest guys didn't. You see that happen for enough years and it eventually does sink in that regardless of what a recruiter proclaims their great travel benefits to be; they are NOT all the same.
When I left the regionals being home based was one of the key factors in selecting where I went to complete my career. QOL is just immeasurably better when based at your home.