Im pretty sure it has to be IPB it has over 2,000 downloadable themes and mods in the marketplace if my memory servers me right.
I would say ipb and vb too judging from the time they have been around and the amount of people who have submitted mods and such
I would say ipb and vb too judging from the time they have been around and the amount of people who have submitted mods and such
IPB has a great variety but i would say phpbb has the largest database of modifications as it has a huge base of developers who have made mod's for it. Just check out phpbbhacks and all the other mod sites
IPB I believe. What's hard is when people only host their plugins on external sites eg there's thousands of MyBB plugins elsewhere.