Darhk by a longshot

His deliberately understated appearance is unnerving. He's a character who could potentially kill everyone with ease he comes across yet all he looks like is a well tailored man.

Dark Archer.

Merlyn easily, too bad they gave his costume to the entire LOA and made it generic.

Merlyn by far , followed by Prometheus.

Ra's then Merlyn

Merlyn for me. However, CW really screwed it up after the first season when they started giving every generic assassin the same outfit.

Merlyn so easily.

Could have been Deathstroke but they made his mask look like a toy.

Slade.

Shouldn't all these Arrow threads go in the Arrowverse forums?

OT: Deathstroke.

Slade I think.

Merlyn and ras.