Dalian, Liaoning, IMO.

Coastal 4-tier city (6M residents), windy year around, 'sunny' is regular occurrence.

I didn't look at aqi.com, but I lived there almost whole year. I think, anywhere in China 'windy' is very important factor, becasue filth in air (and elsewhere) is almost everywhere, somewhere stays on whole day, and elsewhere wind blows it away.

I don't pay too much attention to air, i.e. no face filters or any purifiers, windows open whole day and night. I live in free utilities apt..

I didn't see any foreigner here using face mask, and we most likely have the worst PM index in China.

I guess, only bank robbers and medics are dressed like that in West.

See map of China for lowest PM numbers:

http://aqicn.org/map/#@g/9.6241/30.2344/2z

Cant beat sunny Xiamen - winter being nicely tolerable between 10-20c depending on the weather 

Tibet, Xinjiang, Qinghai, Inner Mongolia, basically the places with the least Chinese people.

For cities I'd guess Sanya. Hainan has very strict pollution laws to preserve tourism. I was there for a few years and didn't once see any noticeable pollution. I believe Haikou even came top of the governments least polluted cities list. I'm sure some places in rural Tibet or Xinjiang officially have less, but for developed places Hainan will be hard to beat.

I hope Shenzhen

The Yorkshire Moors. I know, not 'officially' part of China yet, but they'll get round to it, you'll see.

Hainan always scores very well on the AQI charts. The coastal areas of Guangdong from Zhujai to Zhanjiang also have pretty decent air quality. Probably as icnif says, due to coastal winds 

My ex-girlfriend's purse. It would take laser technology to penetrate the seals. No chance whatsoever of contaminants entering it.

Zhuhai sometimes markets itself as "one of the cleanest cities in China".... it is probably cleaner than Paris, but certainly a cesspool next to any Scandinavian capital (these being similar in population to Zhuhai)