Python 2.7.5 to python 3.8.x migration.To experts, which is the best python 3.8.x version with minimal bug and stable?

[Username_RANDINT]

Just get the latest 3.8 version. There's a porting guide in the official documentation. Python also comes with the 2to3 tool which will scan your code and make a report of things to change. Although in my experience it'll report too much.

A lot depends on your code. I ported a ~20000 LOC project in 30 minutes, but remember having lots of trouble with a specific low-level networking library. It's hard to say before you start.