The Gladius was made to cut your enemy to pieces and thrust.
I think the Katana would have to be the best.

Some are for stabbing, some are for slashing, some chop, etc.

One of the most impactful in changing combat tactics was the sabre, used for mounted cavalry (cavalry used to have barding and the tactic was to steamroller foot soldiers, the sabre allowed mounted troops to run through and break up battle formations), but one of the longest in historic use was the short sword, the most famous and arguably ideal design was the Roman Gladius.

The Katana was a wonderful, beautiful weapon, but it’s long arduous time to manufacture and expense made it a class or caste based weapon. In battle, only the elite had katanas – decades of training was required to master the weapon.

For practical purposes, a weapons’ utility also has to be measured by a few additional factors: its ease in manufacture, its ease of distribution to the largest number of troops trained in its use, etc.

My vote is for the Gladius, as a soldier could be trained to use it effectively in a couple hours, efficiently in a few weeks – it was easy to manufacture and distribute. It could pierce, chop, slash or bash and used with or without shield (long sheild, buckler, kite or tower, flexible in its use and ulitilty in open or close rank, and can be slung to free up the soldiers hand without creating a tripping hazard for himself or others.

However the ones wielded by religious fanatics (the crusades, spanish conquistadors) always seem to spill the most blood.