The best supported Ubuntu Touch phone is Pixel 3a. One of the core devs owns one and is actively invested in getting all the features to work, get decent battery, hardware acceleration, etc. Other phones may have various compatibility issues, like lacking GPU support, unreliable GPS, no USSD codes, etc.

Be weary of 16.04 devices, they have old manufacturer kernels and can't be made to work with newer releases. Basically anything than ran Android ~4.0 at release is off the table.

PostMarketOS actually has a list :) The one I see most often touted and used on the Mobile Linux corner of the Fediverse is the Oneplus 6 or 6T, but I'm happier with my Pinephone Pro (even if just for the swappable batteries and easy repair)

https://wiki.postmarketos.org/wiki/Devices