Many believe that his unification of the Mongols — rather than the conquests that he initiated once he had unified the Mongols — was Chinggis Khan's biggest accomplishment.
Unifying the Mongols was no small achievement — it meant bringing together a whole series of disparate tribes. Economically the tribal unit was optimal for a pastoral-nomadic group, but Chinggis brought all the tribes together into one confederation, with all its loyalty placed in himself. This was indeed a grand achievement in a country as vast as Mongolia, an area approximately four times the size of France.
Once Chinggis had succeeded in bringing the Mongols together, in 1206, a meeting of the so-called Khuriltai (an assemblage of the Mongol nobility) gave their new leader the title of "Chinggis Khan": Khan of All Between the Oceans. Chinggis's personal/birth name was Temujin; giving him the title "Chinggis Khan" was an acknowledgment by the Mongol nobles of Chinggis's leadership and their loyalty. From that point on Temujin would be the Khan of all within Mongolia and of the Mongols.
For more on Chinggis Khan's unification of the Mongols, see:
• The Mongol Conquests: Tribal Group vs. Mongol Unity under Chinggis Khan