Abdül, you leaned the wrong way, but I am gonna give you the rep and the victory anyway, because you clearly knew who I was talking about.
The answer is indeed Ögedei Khan. Almost no one knows about him, but it was under his rule that the Mongol Empire reached its furthest extent. And it was his death that most altered the potential course of history, because, as Abdülmecid pointed out, the Mongol armies turned home to attend the Kurultai (the gathering to elect a new Great Khan). At that time Batu and (the criminally overlooked) Subutai were crushing their way through Europe, and by most scholarly accounts had routed the last proper resistance that there might have been. Thus, Batu and Subutai were in a position to finish Poland and Hungary, and then cut clear through the HRE and France, all the way to the Atlantic. It would have meant a Mongol Empire from the Pacific to the Atlantic, and would have led to a massively different Europe in the centuries that followed. But Ögedei died, and without him, the Mongol generals had no proper writ of authority, at least not until a new Khan was elected. So they turned around, left Europe in ashes, and then crossed back roughly 3000 miles to the Mongolian steppe to have their Kurultai. So if Ögedei had not died, we might all be Mongols right now
Anyway, Abdülmecid has the floor!
EDIT: If someone would be so kind as to give Abdül a rep on my behalf, I'd much appreciate it! The lad did well, but apparently I have already praised him in the recent past, and cannot do so again yet.