I've always pretty much used IPB (I should type IP.Board) it's what I originally started using, as I was never a fan of phpbb. I would say it's very difficult to single out a certain one however because I've used so many and appreciate them in different ways but I generally stick with IPB and MyBB.
Undecided but MyBB is very powerful and as a great community, which I've been lucky to make so many friends from. (unlike IPB) who have so many stuck up users who don't really believe in community spirit.
