Japanese swords in general are heavier (more dense), so they cut better than their European counterparts. But because they are "unbendable", when used against something hard like plate armor, they lose their edge very quickly and sometimes just break. European spring steel swords, meanwhile- they would come out perfectly fine.
European swords of a comparable weight also have more reach.
So no, Japanese Katanas aren't the best swords in the world. They're great against unarmored opponents because of their sheer cutting power, but not that great when there's armor or even another sword involved.
They look damn cool though.