For me Cialis lasts about 72 hours, and Viagra 12. That makes Cialis a clear winner in my book. Plus less side effects like stuffy nose, pounding heart and seeing flashing lights (which happened to me with the V).
V has a bit more of a powerful effect than the other two, but not enough to matter. I only tried Levitra a few times, but I think it lasts a bit longer and has less sides than V, so would come in second place.