Aren't katana the best swords?

That depends. Simply put the answer depends on too many variables to fully list here. Focusing on a specific blade is not advisable. It ignores the wide variety of blades, materials, goals, uses, techniques, situations, tactics, strategies, armour etc. 

Nearly all swords are carefully engineered for one thing: killing. Swords were extremely good tools for this objective. No matter their origin, shape and other characteristics, swords are marvelous feats of engineering and metallurgy.