Zweihander, using the prestige class dedicated to their proper use in Swashbuckling Adventures. The 7th Sea version had illustrations of the stances, a very versatile weapon in the right hands.
Halberd, using the Pole FIghting feat from the Netbook of Feats. (I have seen one in use, and yes, the haft is a weapon in its own right.)
If using core D&D then the bastard sword, again the versatility.
And all my characters carry a light crossbow, so while it may not be my 'favorite' it sees the most use.
The Auld Grump
*EDIT* Does anyone know where to find the clip of a Swiss Vatican guard using a halberd? In one smooth move the guard brought the haft behind the fellows knees - tripping him, and the blade to his throat as he went down. Believe it or not those guys are still trained in how to use those things!