I would say Changping is the most important battle of Chinese history, because it paves the way for Qin unification. Xiangyang is important too, but I feel that the Mongol victories over the Jurchens are even more significant than it, because blocking the Mongols from conquering North China would likely ensure the survival of Southern Song.China's a bit difficult, given its long history, but...
I think by far the most important battle in Chinese history was the Siege of Xiangyang from 1268 to 1273, but that ended in the Sung Dynasty getting defeated by Mongols so I guess it doesn't count.
Land battle - Changping in 271BC removed the last obstacle to Qin dominance in China and so in a sense fostered the creation of the first Chinese Dynasty.
Naval battle - Poyang in 1363 ensured Zhu Yuanzhang's control over China under the Ming Dynasty, and given the immense changes that Zhu Yuanzhang made to the Chinese government (imho not for the better), this is probably the important.
I don't think Lake Poyang is the most important naval victory, though it is certainly significant. I feel that one of the Song victories over the Jurchens (Tangdao or Caishi) would be more important because those kept Southern Song independent. Barring that, the battle at Chibi (Red Cliffs) would also be a contender, for keeping China divided during the Three Kingdoms period.