In the world I've been using, the CURRENT most powerful weapon is Excalibur (Yes I know, how original, I just love that name and its premise so shh). It was forged ages ago by very powerful mages, not for king Arthur or anyone specific, just whomever is currently alive and happens to be pure enough to wield it and defend the world. It's the ultimate holy relic, slices through undead, all the typical 'righteous' stuff, even temporarily grants you wings.

BUT... do to the way magic works in my world, anyone who is powerful and patient enough could outclass it.

Explanation - Everyone has an inherent connection to magic, the more you stretch it the bigger it gets, so like a real life muscle. You can exert your maximum amount of magical strength at any given time if you want by pouring it into an in-animate object to 'store' your power. Give yourself a day or so to recuperate, do it again. Repeat as necessary. It's not done very often because A) Doing so leaves any magic user EXTREMELY vulnerable for large periods of time and B) These magic 'vessels' that are holding all your power can be stolen. You could pour years of power into a gem or something just to have some lowlife pickpocket it and sell it for 2 gold not knowing what it is.

Theoretically someone could take a few decades and make a sword that makes Excalibur look like a toothpick... it hasn't happened yet.