vbulletin 4 till I drop dead!! Why? Because I tried vbulletin 5 in the lab and found:
1. Didn't like the way it looked on front end
2. The admin panel was heavily lacking in controls
3. It seemed to be more a "Facebook clone" than a real forum suite
4. It was a paradigm shift for me on both sides. How to support my users if I can't make heads or tails of the front or back end of the site?
5. As already said vbulletin 4 has huge plugin and skin following unlike vbulletin 5