[GSM]

I’m easily saying Duelist and Martial Artist, for sure.

Duelist because it covers so many bases, as One-on-One, stat add-ons, the access to devastation via Voltiger crits, extreme mobility, a wide range of damage types, etc.
Martial Artist simply because of how much utility it gives just by having it equipped. Immunity to knockdowns, essentially, free Evade (and more against Bow users), bonus damage from combos, mobility and damage with Shukuchi, even self-healing and status removal.
A close runner-up would have to be Archer, actually. I think the bonuses Archer gets and their rather vital skills with crippling are quite devastating if they land in line, able to gimp builds quite easily.