vBulletin, if your going to spend money on your forum. If you ain't I recommend phpBB3 with a free web host like hyperphp or MyFF which is a forum host which have customised it so that you don't need a host any more you can just create web pages in the ACP.
On the free end I'd say MyBB or SMF. On the paid end I'd say InvisionPower Boards. The new Admin interface seems a little dumbed down on IPB3 seems a bit of a turn off. I prefer IP Board to vBulletin on the whole, but others will differ on that.
I've tried ProBoards, InvisionFree and Zetaboards, and I prefer Zetaboards hands down. Though if you are serious about your board, don't get a freehost. If you want to cut costs you can even get a free software (I mentioned MyBB and SMF above).
Then again, paid software offers better support and more features. Free and paid software is like chalk and cheese. It's up to you to decide if 150$+ a year is worth it 🙂