Whose? Gul'dan's? Well, he was the leader of the entire Orc horde, which would make him arguably the best fighter (though Durotan, the one clan chief, held his own well), or he would have been overthrown before. And the fodder Orcs were far larger, tougher and stronger than normal humans. They were literally caving their armour in and sending them flying with individual hits (and were even strong enough to throw warhorses around). The only humans (non mages) who were able to genuinely match any Orcs one-on-one were Lothar and King Llane. The other soldiers needed to get lucky, use their superior ranged weaponry, or gang up on them in like 3s and 4s, to put them down.
But what makes Gul'Dan so dangerous is his Fel magic, which allows him to just drain the life out of anyone in his vicinity. In the beginning of the film, there are cages filled with dozens, if not hundreds, of slaves, and he drains the lives out of all of them simultaneously, in order to power the portal he was using to send the Orcs through. He also uses the power to kill Durotan in combat. No one in the film showed any counter to this power.