I'm going to vote for IPB too. It's a very good premium software but I think that the price there is a bit high. As a free option MyBB would be excellent. Just need to install the MyAlerts plugin and find a good theme.
I'm not a fan of IPB, to be honest. I just feel that it's a bit too bulky and large whereas a software like XenForo is great. But if I had to choose between IPB and MyBB, I would probably go with IPB. I think you just get a different feeling with a paid software and it's better to have it than MyBB.
From my experience IPB is much better from MyBB, I do use MyBB, for me it lacks a lot of functions and the functions it already has are mostly worse from similar function in IPB. The great thing about MyBB is community, you can find plenty of addons and styles for your forums, MyBB community is bigger than IPB, since Mybb is free and they make every public addon for free, in my IPB community I noticed few paid "hooks" which is not a problem for me because everyone wants to earn something for their hard work. But overall IPB wins for me, I don't use it for that long but so far experience is great