Lord almighty VB5 is a pile of crap. VB4 is great, except for the spam problems. I administrate another forum that runs VB4 and we have the largest problems with spam, it's just numbing!!
VB3 was great back in the day but now its way outdated.
I like SMF a lot. It's free, and actually works pretty well. I also like I.P. Boards. Think that software works pretty well too aside from a few glitches.
I say keep SMF and just keep trying to fix any issues. I haven't found really any yet that are concerning enough to ask to be fixed.