I have aNexus-s v 2.18. I am wondering which version of Prog is best?

2.18 is not a Nexus revision, but a version of the Technotrend or hauppage software / driver package.

The hardware revision is printed to the card itself, in many cases on a sticker, but 100% sure in copper directly on the board base material.

However, there is not a perfect version for all purposes, even if just thinking about SS1 / Nexus.

Your location does matter, as some versions did not scan well in America, some other in Europe.
The Nexus version does matter, as a 2.3 won't work with really old drivers and by this might fail with similat ProgDVB versions.

Your needs do matter, as if you want to use the TV out mainly, want to play oput recordings there, or if you need de-interlacing for the PC screen picture.
Many more aspects like those...

However, as you are owner of a Nexus and can have as many installations of ProgDVB in seperate folders as you like, as long as you don't install yet another demo codec each time...
Obsolete versions do not require de-installation, it's enough to delete the folder structure. The uninstaller in Windows system control -> software will only know the last one installed anyway.

Channel data bases often are not down compatible with older versions.

Keep recordings outside of the installation folder structure, best to do is, define an extra place.

Have fun experimenting, reading, searching and, of course, watching TV.