If you want to meet characters, see great theming and see different things to WDW, then I would say Tokyo and Shanghai. But, visit Shanghai first and then visit Tokyo.
Hong Kong is a lovely park, which is normally pretty quiet and it does have some unique attractions, including Mystic Manor and the new Iron Man ride. But, it's pretty similar to Magic Kingdom and the surrounding mountains look cool, but they're not purpose built "theming".
The theming at Shanghai is great (although it will improve once the greenery has grown a bit more) and the theming at DisneySea is astounding.
There were a lot of character meet opportunities in Shanghai and some of them were really quick (e.g. Mickey Mouse, Captain America, Spider-Man and the Star Wars characters). The other character meets can be tricky, because they don't abide by the signposted meet times.
Character meets at Tokyo are okay, but popular. They do seem to have quite a few "walking around" characters if the weather is good and that can be a good opportunity for a character meet, depending upon the crowd levels (e.g. trying to get to Duffy or Marie would probably be impossible).
I think Shanghai and Tokyo are also both the most different to WDW.
BUT, both Shanghai and Tokyo will present their own challenges. Even with full knowledge (and prior visits to China), I found the cultural differences at Shanghai to be confronting and aggravating. I was also really disappointed in the poor attitude of the cast members and the incredibly poor communication, even with their published and signposted information (e.g. incorrect park opening times, incorrect ride opening / closing times and incorrect character meet times). It can also be quite crowded, which is the main downside about Tokyo (although I'm not sure of crowd levels in mid-December as the colder weather may keep the big crowds away).