SMF gets my vote, because it is eay to modify and installl...but then again, I have only ever tried SMF....so it gets my biased vote.
Previous threads basically say that phpbb and smf are two strongest alternatives. You will see that some people say that phpbb is very unsecure and open to spam. But you will see the best picture if you search for the previous threads! You can start looking at "Similar Threads" at the bottom of this page.
Search, good idea. Just tried it and found a thread showthread.php?t=29088&highlight=free+forum+software Alot of usefull info there not just a newbie being slated.