You can choice between these boards

Invision Power Board 1.x
Invision Power Board 2.x
PHPBB
vbuletin 2.x
vbulletin 3.x


My Vote:
Invision Power Board 2.x
But Im starting to use IPB 1.3.1 lately because you need to pay for 2.x now

Oops I forgot This new one Simple machines Forum

Hmm.. yet another "which forum is better thread" ... (not to be too overly sarcastic...)

http://www.webhostingtalk.com/search.php?s= It does wonders!

But vB 3

vB 2. (must add some more text!)

my vote goes to vbulletin 3.x

I honestly like IPB 1.3.1, and think it's the best, but vB's good too.
I liked phpBB before, but noticed that it's more likely to get hacked or spammed than IPB or vB so it's not the best for sure.
The above is my personal opinion, some people would disagree.

I've used invision board, vb, and phpbb. Have to say had the easiest time using vb.

i love phpBB , but i more like the options that VB 3x have :)

IPB - dont like that one

Simple Machines is actually a good board, and integrating it into your site is.. easier than any other forum I have come accross.

i like vBulletin. but ipb has a lot of mod/hack than vB and invisionize is more organize than vBulletin.org

I use phpBB, for the sole reason that I am used to it...

Inertia, resistance to change ;)

Vbulletin 3!

clearly vb for me

vBulletin 3.0.x - I just love vBulletin, the features are awesome, especially on the administrative end of things, as is the support over at vBulletin.com, and vBulletin.org is FULL of USEFUL modifications.

VB3 if you have the money otherwise PHPBB is as good as it gets for free. Besides coding your own :)