I think personally vBulletin is the most powerful and also a good choice if you are
making a forum for real, and not just for fun or small projects.
If you want a small forum with less power, but still a lot security you should go with
SMF seeing as those are quite secure as well. The thing is, as long as you are up to
date 100% all the time, or allmost, and doesn't install 1 million addons, or check the
addon-codes yourself, then your forum should be safe.
I've had my forums for 6 months now, and i've had loads of attacks and pentests on
it, due to some people just think it would be funny to look if something was not coded
correct, or some just wants to see if they can hack my forums.. And so on...
The good thing is i haven't seen that anyone have hacked my forums yet.
phpBB, no, no no no no.. First of, it's slower than SMF and less powerful than vBulletin.
phpBB is simply not a good forum to run, in my oppinion, and try go to milw0rm and
check out how many addons and PoC's have actually been released about phpBB, it's
seriously insane that they have made that many code-flaws.