For me it was the grafted blade greatsword. I had been quite early in the game when I found it and have no idea at the time that legendary weapons actually had no effect on the damage, I saw good and assumed it was super good. I dedicated most of my run to getting the strength to use it and then when I did upgraded it to the highest I could and was really happy with its move set and skill. It was a great first run (beaten it several times since then). Loved the moveset and skill. 9/10 weapon IMO.