I started with 5, then got 6, 7 and now run CS. It's a toss up between 7 and CS, probably because quality and productivity improved with them along with a better grasp of the many features. I love the Actions.
v 1.0, that brings back some god memories and also SuperPaint as well. I still have PS v 1.0 on one of my CD's if you are still interested.
For me version 3 was when I got a full handle of all the tools and some very quite developed since v 1.0, that being said I prefer v5.5
6, 7, 8, 9, and it shall continue are all nice however it started to be way too bloated after 5.5 and load time started to drag along with other hardware requirements.
If you wanted to counterfit US Dollars then stick with version 5.0 or earlier. (I'm joking.) Really, the US Treasury mandated that all versions after 5.5 or 6 (somewhere around there) have built in mechanisms to detect when any image of US currency was opened.