i've tried a lot of them.. starting with the smallest..
first i tried phpBB, which i thought was pretty nice at first, but i had to keep modding it myself and downloading all those mods for simple functions... also one of the most annoying things is that it doesn't show the latest topic's name on the main page.. it just shows the last poster's name, not the thread. it was very very simple, but it was nice. modding it was fun too. then i decided to leave.
SMF, though i haven't worked with it from the admin panel, sucks in my opinion. i don't know why, but i just didn't like it when i first saw it. i even left a few forums on which i was a regular member just because they switched over to SMF. and from what? from VBULLETIN!! AHHHH! SMF.. it's just too... ughh...
vbulletin is nice. i've worked from the admin side for a while and it was pretty impressive with everything it had. it basically has almost everything you could ever need, and best of all is that users are very very satisfied with it. it's just great.. though when i was on xtremevbtalk.com i though vbulletin was a little bit too slow.
my all time favourite would have to be invision power board. the latest versions (you know.. the ones you supposedly have to pay for), not the old blah ones. it's just slick and sexy. i worked with it for a long time and it basically fulfilled all my needs. it's also very customizable, which is just sweet. it looks very nice and smooth and the admin panel just kicks away any other. i love it.
there was other forum software that i worked with before all the above but i don't think any of those would be worth mentioning
