Hey there! Check out this thread!

Short answer: there wasn't one. That's one of the major changes that the eventual victor of the Warring States - Ying Zhen of Qin AKA Qin Shihuang - actuated with his unification of the written and spoken languages, as well as weights, measures, etc...

Not that it really stuck, because CHinese people still don't speak some single unified language if they can help it. Though they're called "dialects, the regional variants are much more akin to distinct languages within a family (ala French/Italian/Spanish) than anything we'd really think of as "dialects".)