Great - 7 responses in the thread and not a single person has addressed the original question!

The best cabin will be the one closest to the centre of gravity of the catamaran. That CG will be pretty close to where the mast meets the cabin sole in the main cabin; but there's no cabin there. So the least motion will be in the starboard center cabin.
Note that on a catamaran the differences between the cabins are going to be negligible and most likely not even noticeable, so the best choice of cabin will be the one with the best (most) ventilation and that might be a forward one; again on the starboard side in order to be away from the crew cabin - those guys have to be up and about at all hours when on charter.