I'd like to attribute more to the culture as a teamwork rather than name "the Greatest" of certain person in certain peroid of time.
Thus, my stand,
That Qin ShiHuangDi's unification of China is a turning point for the culture despite his ruthlessness.
Han WuDi's advocacy of Confucianism, and military expeditions against the XiongNu etc was a stage marked the further consolidation of the culture that it inherited from the previous short-lived dynsaty. Which of became the core of the Chinese culture.
Consider the "Era of division"(sixteen kingdoms-Northern&Soutnern Dynasties) marked the end of Han dominant position i.e politically and changes in relation with the semi-or more sinicized nomads from the North. Out of ethnic and cultural conflict, there is 1 man more or less successfully established a style of governing that begin to change the face of this relatively chaotic period, was Northern Wei's Wei XiaoWenDi. Although he never achieved complete military conquest of the South.i.e unification of China.(which he personally so eager to achieve, for he wrote a poem"all lands are under light of sun, except that of the south".Nevertheless, hes cultural achievements(despite his reign was relatively short due to early pass away) was a combination of XianBei military success& importation of Buddhism& adoptation of Confucianism.. this at one end ease the tension between Han&non-Han ppl, and at the other end accelarated the cultural and ethnic acceptance by mutual means. Inwhich ultimately gave rise to Sui-Tang cultural, which was characterized by its open-mindedness and multi-culturalism, which was markedly different from that of Han, i.e b4 the collapse of West Jin dynasty, and the begining of divisive period. In conclusion, XiaoWenDi's role and contribution was in a way like that of the unification of China under Qin ShiHuangDi. And he's just the first of the rulers to setup a mutually respectful multi-ethnic state. Which also characterize the current ethnic policy of China.(though disputes can never be ignored completely). So my high points to him.
GengghisKhan was a great figure that has many qualities i would like to advocate. But his "demerit"(though i should admit in a way it's pride for his ppl, i respect their heritages, i.e culturally and ethnically), is also apparent and absence in the following process makes him more belonging to the steppe rather than that of China's concerns. Thus recall whom had more focus on China and did actually had more success and influence, will be his grandson- Kubilaikhan, His number of achievements are very important in Chinese history, consider the decline of Tang power and later the political migration of Song Chinese, the begining of another period of "era of division" for China. Only that the players changed from XiongNu-XianBei(and number of others) to Khitan & Tangut and Jurchens, and to an extend the Mongols. With notably similarities in sinicization and use of Confucianism and Buddhism for maintenance of the authority.etc. KubilaiKhan's Chinese-style Yuan dynasty played a role inaway like that of Sui-dynasty, which unified the divided country into one. Though culturally resistant to Chinese in Chinese's eyes, but Yuan was an extended version of early Qin&Sui dynasty in politically unify the state. And its successor Ming dynasty inaway like the Han&Tang which inherit the previous dynastic achievements, though the Ming is less convincing than the Han&Tang, neverthless, Later, the Qing would combine the assets of Both Yuan&Ming, that is exactly what PRC has inherited from history. (mostly)Anyway, the establishment of Mongol-Yuan dynasty was another turning point for China, despite the Mongols were relatively late comers to affect the cultural progress of China compare to that of already sinicized Khitan&Tangut&Jurchen dynasties. But it was the first of its kind to successfully rule over whole of China. As a consequence it inevitably impact greatly on the culture and it was done under the reign of Kubilaikhan.
Qing's KangXi, YongZheng, QianLong were all remarkable emperors, their rule further consolidated the country. But tension between Han-Manchu never fully eased.
Perhaps, atleast in my mind, the most unquestioned ruler for achievements in certain duetime thats certainly "great" , would be Tang TaiZong. But China doesnt stay in the "Golden era" for ever.
And most questionable ruler for "greatness" is Ming's YongLe emperor...in my opinion. Since, the debates are still on, and the possible future it inspires isn't clear enough for everyone to see.
Edited by Beijingguy