I would start by recommending a device that's got ubuntu 20.04 as an option
Xiaomi Poco M3
OnePlus 5
OnePlus 5T
OnePlus 6
Xiaomi Mi A2
Xiaomi Redmi Note 7
Google Pixel 3a
Volla Phone
Volla Phone X
Sony Xperia X
Fairphone 4
Volla 22
I've mostly heard updates from Alfred who praises the Pixel 3a, and also talks about the Fairphone 4, I know the OnePlus 5 runs wonderfully, and I know you can get the OP5 for pretty cheap.
What's best for you, is up to you, but I wouldn't even consider looking at devices without 20.04 support because you don't know how long they'll last, you don't know if they'll ever get a 20.04 update.
Another thing to consider is, which companys help and support ubuntu touch? maybe if you want to support ubuntu touch, you should consider getting the phones that either give you it pre-installed, or at least show off, or help the developers in some way.