The best type of sword is the one that is best for a particular purpose at a particular time. For example, you might need a sabre for mounted battle, a heavy two handed sword if coming up against people using heavy weapons, a light quick sword such as a rapier for fast moving swordsmanship, etc.
My favourite sword is the Chinese Jian. It is long enough and strong enough to meet most situations, equally good in slashing and thrusting situations. It is double edged so that minimal movement keeps you on the attack. I've used this sword against many other types, and for me, it's the best. It outplays the katana because of quick movement.
The best sword in the world is no use in the hands of an inferior swordsman though. On the other hand, the worst sword will still win in the hands of an expert.
Take a look at some of the information I've written on Chinese swords over at this site www.chinese-swords-guide.com