Welp, that's still fairly broad.
Ere-yesterday I spoke with a guy who's hobby was just that. For about an hour.
So... he daily drives Fairphone 5 with Ubuntu touch, but played around with just about any distribution.
When I asked him about PostmarketOS, he suddenly pulled out a laptop, rummaged through his backpack, finally pulling out a PinePhone, and went on to reflash it from Sailfish OS.

Unfortunately, his laptop suddenly decided it doesn't feel like writing data to any external storage.

I didn't know you can just access the storage of PinePhone directly. That means I could just easily do full backups and not worry about bricking it.
Though he doesn't recommend it due to being expensive and under-powered. Both of us independently agreed on Oneplus 6 being best candidate for PostmarketOS.