The best software that I use is IPB. This is very useful software to build your forums. This software can be optimize as you want.
phpbb, while for a paid software i prefer using vBulletin. xF is a good lookin but even if it`s code is cleaner than vBulletin`s, it is on the second place in my top3, at this time.
I actually like smf as much as I like vb. Very easy to use for those who have little time and just need a community to answer questions here and there.
It is cheap
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