[Slydria]

Depends on what metric we're talking for strongest:

If we're talking raw damage then duelist will easily win this on just the merit that it has Fleur giving them that extra attack, but on top of that, it also has passive bonuses that further increase potential damage like One-on-One, Flottement, Elangreve, Riposte and Poise, as well as the versatility that it's Offensive Skills provide.

However, if we're talking all around then I think Martial Artist would edge them out. It has good damage options (Geldoren, Heaven Kick), great utility (Meditate, Cleanse Body), massive range and mobility (Shukuchi + the Combinations with HK and Geldoren) and can be built for evade or defense.

It's no wonder that Duelist/MA is one of the most popular combos for classes these days.