phpBB has big plugins repository too
There is no such thing as a `plugin' with phpBB.
What you have is patches which you apply to the phpBB code.
They're not even patches as you and I know them, they're a bunch of instructions (open file X, find line Y, change it to Z), someone really needs to introduce these guys to
diff(1).
This is different from vBulletin, which has hundreds of (Documented!) hooks which can be used by plugins.
I believe there phpBB currently some basic code which adds hooks, but it's non-functional, as far as I could find a few months ago there is little headway to making a fully-featured plugin system ... It would seem the phpBB people are actually happy with the current system of `plugins'.
To answer the original question:
In my opinion vBulletin is the best overall forum software, I tested several packages (Including phpBB and SMF mentioned earlier) when I set up a forum some time ago, and found that vB is the best one by far.