[damientheomen3]

As BIGEASY said, there aren't really 'benchmark versions' of OOTP that stick in online leagues the way 6.5 was. 6.5 was the last version on that build of OOTP before the game was remade from the ground up with the release of OOTP2007 a couple years later, and has been building on that version with the yearly releases ever since, so it's almost universally standard for online leagues to run the most current version of the game (or to switch to it a few months after it comes out, once the game has had a few major patches to iron things out and make sure it's stable to use for their league).

And with that, given that there aren't really major differences between versions and its been building on itself for over a decade now, if you'd like to get into modern OOTP you're best off just going with the newest version, 23. Unless you plan on doing some super duper niche stuff, 23 will be able to do whatever it is you wanna do.