Sadly, I don't have a favorite yet. The only device lying around that is supported by any of them is my Moto G4 Play. PostMarket OS has support for it, but last a checked a few months ago, battery charging was still wonky. The wiki actually recommended taking the battery out and charging it externally lol.
For a secondary device (or one that I'd just use as a mobile server), I'd be able to go without GPS and VoLTE (though GPS would be nice for some things like running an NTP server with GPS as its time source). Haven't looked into Sailfish or Mobian, but it sounds like they have less device support than PostMarket and Ubuntu :(