P13 is the latest version, supports both Superfly and Firefly rendering. The two methods diverged enough now that materials made for one don't work well for the other, but all versions since P11 have both. If you like PBR rendering, go for P13. If it does not matter to you, anything from P2014 up may work, except many figures that use more advanced materials and rigging may not work.

It will, also, of course, depend on what type of art you're creating.

I personally still use a lot of old content, just rework materials, it's worth my time since a lot of content built up through the years. If the model is solidly built and has good material zones, assigning new mats or converting it to physical based is possible, it only takes time and/or the right scripts.

What is also important is that there have been at least 2 Python versions since Poser 8, so scripts that worked in Poser 8 bought third party will not work on anything later than P2014. 

(that was the dollar answer, hope it helps a bit)