Syrio Forel clearly. Honestly, i don't know who in this list could beat him. He kicked those knight's asses with a stick! Had he had a proper sword he'd have killed each of them half a dozen times, even with their armour.
He'd be bugger all use in a real battle, but in a dual/small skirmish he'd decimate more heavily armoured opponents. Think bronn & the Red Viper, both of whom won duels against much bigger, more heavily armoured knights (ok, so the viper lost, but he'd beaten gregor, then got cocky. Not something syrio would do). Brain beat's brawn every time, and with a sword i don't think we've seen anyone half as clever as syrio.