really depends where you go, because china is such a big country, for example in hainan sanya during the winter they have their high season but some where like guilin is dead during the winter 

Anytime of the year to kunming, because the weather never changes.  Springtime weather all year.

Depends on where you want to go. Beijing, best time is spring or autumn. Shanghai, the same. Guangzhou, winter is pretty okay. Do not come during the summer, it is hot and humid like hell.

Spring or autumn - in east, central and north provinces You can enjoy nice weather, and landscapes (fresh leaves, or multicolored autumn), and on south is still not so hot.
 

I'd say spring, weather is warm and you can get cheap flights.

Fall or Spring if going to the Guangdong, like Guangzhou, Shenzhen, or next door in Hong Kong.  School is in session, and weather is typically nice, so the tourist spots will be easy to access and not crowded.  Try to do everything on weekdays when everyone works or is in school.

Winter can be a bad time if weather gets too cold, and Summer is oppressive.  Plus, the holidays are the only time of the year the locals vacation.  Never vacation during the Mid-Autumn (late summer) Festival or Spring (actually in winter) Festival, or you are going to have a bad time.

Be careful if considering Nanning / Guangxi in winter. Winter is the 'dry' season so much less rain (which can be almost daily in the height of summer), and while the temperature only falls to 5 - 10C you have to take into account almost nowhere has heating. 5 - 10C may not sound that cold but when that is the MAX you will get too all day it quickly feels freezing!

In Nanning my favourite is Autumn or Spring, trouble is sometimes those seasons last a whole week!