I think it depends on preference.
If you are able to pay,
vB and IPB are definetly worth the investiment. If you are looking for a starting forum to fool around with, use phpBB3 or SFM. I've heard people using YABB. However, YABB uses ASP and also requires a MSSQL database. If you are looking for a customizable forum that offers a lot, I recommend searching for a modded version of phpBB3.