King Zhaoxiang of Qin
Reigning over 55 years, he was one of the longest-serving rulers during the Eastern Zhou Dynasty. Although making numerous policy mistakes during his later years, his aggressive territorial expansions were pivotal in consolidating the state of Qin as the dominant military powerhouse in the late Warring States period. It was the strategic dominance established during his reign that paved the way to Qin's eventual successful unification of China under his great-grandson, Ying Zheng.
Warring States period
In 221 BC, Qin conquered Qi. 
Qi was the final unconquered warring state. It had not previously contributed or helped other states when Qin was conquering them. As soon as Qin's intention to invade it became clear, Qi swiftly surrendered all its cities, completing the unification of China and ushering in the Qin dynasty. The last Qi king lived out his days in exile in Gong and was not given a posthumous name after death, therefore he is known to posterity by his personal name Jian.