Well, I may have MY favorite, but that doesn't necessarily make it the favorite of others, and so, it really all boils down to what features you are looking for, what you are hoping to accomplish with your forum, your experience and also, personal taste. I doubt that anyone can answer this question for you as we have no idea what these things are from your standpoint. I know that I don't, and so, I am not going to act like I do. You'd have to ask these questions of yourself (and possibly your staff and members in terms of what features would they like to have available, and you'd have to ask your staff what features would allow them to do their job as efficiently as possible, and while, bringing up possible worst-case scenarios that could occur on a forum, where your staff would have to handle it, and you can ask them what they'd do in that situation) and then you can base your decision on their answers, as well as the ones you'd have to ask of yourself.
Personally, I am for IPB, and always will be. Simply because of what I've stated above, and also, because of the fact that I have used all others, and found it to have everything I am looking for, and would need to run my forum as efficiently as possible.
I started out with PB, and I quickly grew out of it. I tried PHPBB, and quickly found that it was missing features that came standard with IPB (such as a split/merge feature for topics) and the appearance wasn't all that great either.. and so, I went back to IPB software, as it had all the features necessary to aid in running my forum, (such as more than 35 measly places to put banned words, as well as the fact that you'd still need JavaScript to have different images for each skin you had in your skin changer, and the lack of a portal, to act as a homepage or the like, without the need of a HUMONGOUS code which would undoubtedly conflict with other codes you may have already.. etc.. all of which, are gripes I had about PB) IPB had a Mod CP, and many more options for both members and admins alike. IPB also had some features to give my forum a few extra perks. Such as the ability to post flash games and videos using [doHTML] etc.. (which, I don't need now as I have custom BBCode.)
Although ProBoards lacks many, many features, it is good for beginners. It was the first software I had used, and after realizing how limited it was, and had pinpointed exactly what I wanted my forum to become, and wanted my members to be able to do, and knew that I'd never accomplish that with that software, I knew it was time to move on, just as I have with the other software I had used before finally choosing IPB. I had asked my staff and members about the features they'd like to see, and I asked my self questions about everything else, and the answer to that, was IPB.