Tony Vargas said:Don't have any FR stuff, but there's a really big hammer in the DMG: the Maul of the Titans. It counts as a +3 GreatClub.
The only reason I can think of as to why greatclubs aren't simple weapons is that they didn't want all clerics and sorcerers running around with them.
Other than that, they are inferior to greatswords and greataxes in every way (except price, of course

I think a martial, two-handed hammer should compete with its peers. So that is why I set it at 1d12.
But to each his/her own. YMMV.