A hemisphere, I think, would give the best results.

The reason being inertia. The hemisphere would have the largest mass of water moving at the maximum radius and maximum depth. A cylinder of the same volume would be either deeper but narrower, or wider but shallower, hence have less inertia for a given vortex velocity (unless it was very shallow and very wide, but then the surface friction would be huge). Again, a hemisphere has the lowest surface area for a given volume, hence less surface friction. A cone would probably be the worst of the lot.

Mind you, I'm making it up as I go along, so I may be completely wrong

pca