IWhat is the best weapon in Mabinogi

This depends ultimately on two things:

1) If you choose to be mostly using Melee, Alchemy, Magic, or Life Skills.
2) Your playing style.

There is no best weapon for Melee players, it is ultimately based on if you prefer damage rather than speed, or vice versa.

An Alchemist's best weapon is a Cylinder, only sold in Tailteann.

A Magician's weapon can be found from Lassar, or the Healer in Iria.

There is no weapon for users of Life skills, but use a tool for the appropriate skill.

NOTE: There is no job advancement in Mabinogi, just skills that you can choose to ultimately follow.