Deadliest sword? My Hanwei/Paul Chen Practical Pro Katana!!! Actually thats a cheap chinese copy, but still real functional carbon-steel sword for light martial arts usage.
Katana is perhaps most overhyped sword and full of misconceptions. Yes, IMO its most beautifully crafted sword ever, everything in it is a piece of art like Hamon line and Hada pattern, gorgeus silk ito wrapping, everything looks perfect in it.
But there is a lot of hollywood crap around its superiority.
Yes, its differentially tempered, meaning it has hard edge and soft back, and it adds a beautifull cloudy line to cutting edge, a Hamon. This method allows very sharp edge without it shattering to your eyes, but edge is still brittle and blade can bend easily under hands of rookie after accidentally botched cut.
And it only has one edge, so versatility is limited. Also being curved, reach is also more limited than with straight sword. Well, being curved it has very effective slicing motion when cutting. Cuts thru flesh and leather like butter if one knows how to strike right.
It is also folded. A steel was folded over and over again, even perhaps different hardiness of steel was folded over another. This creates a very cool looking wood grain and burl pattern to blade. However, both of these techiques were not unknown for western blade smithing culture. Why Japanese used these? They HAD to. Ironsand in Japan was VERY POOR quality, so these methods had to be used to produce serviceable sword. Fold hammering process hammered and evened the impurities away and combined with differential tempering produces a usable sword. No magical knowledge, just necessity. In western swords these techniques, while known, were unnecessary cuz steel was much cleaner and folding would have provided nothing, just aesthetic beauty, (and there are such swords in west too, like folded "Viking swords" and damascus blades and so on) and differential tempering works only in single edged sword cuz that process forcefully curves the blade. West preferred double edged straight ones.
But i still love it because of its beauty. Close second would be Bastard/Hand-and-half sword.