I have to say that playing with the fitting room last year was the most fun I've had with a prog in a long time.  But then I had a HD crash and lost 8 mos or so of my wips (pretty much everything that I just didn't think was finished enough to back up).

Every version has been fun for me in its own way. 

Poser 4 was the first I used, so it was fun just posing and screwing around. 

Poser 5 added Firefly, and that was a lot of fun to play with as it really broadened my understanding of rendering.

Poser 6 was fun because I liked the figures included with it, and it seemed much more stable than 5.

Poser 7 added the morph brush, which I would not want to live without now.

Poser 8 improved the OpenGL interface, and I really enjoyed seeing things look closer to their rendered appearance.  Enough to spend more time with it than I had been.

Poser Pro was just so much faster I went started attempting things that I didn't have the patience for before.

PP2012/14, just so many little things made everything better.  I had seriously considered jumping ship a few times, but now I can't see that ever happening.

For me, no version has been any more "fun" than the others.  Some versions OTOH were definitely more frustrating.  I get more enjoyment from using Poser the more I learn how to do.  If it never evolved beyond where it is today I would still enjoy it more each time I upgraded to a faster machine.  In the last few years I've undertaken some fairly time consuming projects.  I look forward to having the time to just "have fun" with Poser again.