Garrosh, from proud Warrior and military figure given a chance at leading the whole Horde itself by Thrall, because we all know Thrall was a laughable leader that would kill the Orc populace if he kept charge. So that proud Orc has a good start, doing what is good for his people and becoming a popular leader. Not too long after, without even being too provoked, he desires to destroy the Alliance to every life, and alienates from the advisors Thrall gave to assist him, like Vol'jin. He then goes on to prove Orcs are indeed pitiful at leading by using dishonorable and horrifying methods to beat the Alliance, kill or beat his own people for not kneeling before his will, and is now little more than a Sha infused tyrant. Garrosh isn't the first though, aside from Doomhammer only, most Orc leaders have proven to be either stupid (Thrall), or just plain malevolent (Blackhand, Gu'Dan).
It's why I believe after the Siege, no Orc should ever be allowed to lead the Horde again, because they simply can't do it.