Not the best sword, but among the better ones. I prefer a smallsword, as I am a fencer. However, considering how the katana closely resembles the willow leaf saber, it is, in my opinion, the best sword because you can directly transfer saber techniques to a katana and kenjutsu techniques to the saber. You are probably thinking of the katana from an aesthetic point of view, but some of us see it from a martial artist's perspective.
There are those who talk about combat and armor durability down below-but these people, from my experience, are mostly HEMA enthusiasts, not true practitioners. They can discuss how a blade is made and what materials are used, but their tactical wheel often consists of fewer than two movesets. You will see these individuals relying on one-trick pony attacks in most of their bouts. From this standpoint, I don't see these kinds of people creditable to talk about "fencing" when they can't fence. They know who they are, they know what I'm talking about, and most likely they will give a counterargument to me saying otherwise but I won't care lol
All weapons are made differently, and some techniques are better executed with specific weapons. The shaska, for example, is excellent for flowing cuts and slices. No other weapon offers this particular fighting style. The same applies to the smallsword, katana, liuyedao, and so forth. This is not exaggeration; it is simply a fact: the katana is that effective at what it does.