I would also reccomend UBB.classic. The WWWThreads/UBBThreads 5.x series is a bit too far out of date, and UBB.classic 6.7.x offers many features that the WWWThreads 5.x series offers, the main exception being a threaded display option. There are some features that UBB.classic has that the UBB.threads 5.x series lacks, such as user-changeable display names as well as more advanced polling features.
Keep in mind that UBB.classic uses session post tracking rather than full post tracking.