Right now assassins suck at everything but with DA DD they are the best emp breakers in the game. So if you're truly looking for a best breaker build ONLY then you are interested in a) attack speed (emp takes 1 damage per hit no matter what so you must maximize hits) and survival as everything is going to be casting on the emp to kill you.
Something like:
80-99 AGI (you just want the fastest ASPD possible)
80+ VIT (Ideally you hit 100 with job/gear to get status immunity)
STR/DEX only as critically needed for leveling
The upside is you can find yourself at an emp with 2 dozen other people and you will get the last hit on the emp very very often (at least that has been my experience in WoEs so far) and you can break an emp solo incredibly fast. With all that VIT and the right gear(poo poo, switching armor/garment) you can survive on most emps until you break even with no cranial. Weapons are cheap! Dual fortune swords or dual quad female thief bug MGs are all you need.
The downside is the only impressive thing you can do outside of WoE is vanish any plant ridiculously fast. You attack very fast but entirely rely on your weapon for ATK + lack the dex for most high level mobs.