I changed to vBulletin, although that was in the days of 3.x - Had I been greeted by version 4 I would've gone to something else. XenForo is looking bloody good now.
After a while of trying them, you will absolutely hate both MyBB and SMF. You will choose phpBB in the end, it's the best!
Sounds like a possibility. btw in phpBB is it possible to set the option to let the OPs moderate their own threads? That is the first feature which attracted me to SMF. Thank you.
I use vbSEO so I probably won't change, it depends on if I can get XenForo to forward the vbSEO links to the right threads, if I can't then I'll stick with vBulletin, we have a lot of incoming links and I don't want people being greeted with 404 errors.
They will, all vBulletin and vBSEO URLs can get 301-redirected over to Xenforo's URLs when it comes out.
I have tested this on my live site. vBSEO + vB4 converted to Xenforo, all seo'd links 301'd to new Xenforo firendly URLs. It works great. dodgeboard.com/forums (vb) dodgeboard.com/community (xenforo)
Regarding the OP questions, I think the word best itself has different meanings for different persons, thus the best script for you would be the one which is able to do most or all of the things you need to do. You need to test the three scripts you have suggested by yourself and see which one is best for your needs.
phpbb3 is the best and now they include some additional security features. I recommend phpbb bcoz it is more popular and it has more themes and easily customizable.
Kier and Mike have done a really good job in capturing the nostalgia in why we post at forums, I must say. He is one of the most advanced thinkers in this business.
After deciding to move away from vBulletin, I have purchased both xenForo and IPB. Now I like them both but for my current site I've found that IPB is most suited and so I will convert to that. Don't get me wrong, xenForo is excellent and I will use it for a new project (although not sure what, anyone looking for a partnership then please PM me) but like I said for my current site, IPB is what's needed. I would like to add that from the forum script itself, xenForo would have been fine but I needed blogs and downloads which is why I went for IPB.
i'm using phpbb3 right now and i'm loving it. When i do have a question, is shortly answered by google or in their own forums. With a free software, sure has a great support.
xenforo may be a nice software in a year or two, it's only 4 months old and looks like to me vbulletin would be my paid option, phpbb would be my free option.
For free forum scripts I like SMF the best. MyBB is 2nd. I have used VB longer than IPB, so I will say VB for paid. IPB 2nd. This is a never ending debate really. It is a matter of what you like I think! Some people can and will change there mind on the subject.