Most of the half decent hacks for vBulletin 3.0 have been or are in the process of being "coverted" to vB 3.5. Most of the good vBulletin 2 hacks were converted to 3.
I'd go with the latest version... It's very easy to hack (most of the hacks are just plug-ins, so no need to mess around with php files) and skinning is fairly easy too.