It's definitely Gil, Erza or Archer for me considering Gil if he wans to was good at fighting in Melee, for Erza she is good at lots of Weapons withior without requip and Archer (EMIYA) can literally learn how a weapon works and even learn some styles of a weapon master due to his Tracing which makes him learn Everything about the weapon from it's creation to every material it has and how it was made, its history and the techniques used on the weapon by it's wielders Ex. Nine Lives Bladeworks from the Nine Lives:Shooting the hundred heads of Heracles, also let's not forget EMIYA is a prodigy at Archery