Hanna-Barbera, since they basically helped create the studio in the fist place. H-B extensively used the studio throughout the late '70s to the mid-90s, though they gradually phased them out in favor of Fil-Cartoons, Rough Draft Studios and others.
Nelvana has used both the main studio and Hong Gaung on several of their projects* specifically, Tales from the Cryptkeeper, Clifford the Big Red Dog, Beatlejuice, The Busy World of Richard Scarry, Care Bears (both series and films), Eek! The Cat, Garbage Pail Kids, Ewoks, Marvin the Tap-Dancing Horse, Little Bear, George and Martha, Pipi Longstocking, Rescue Heroes, Inspector Gadget, Madballs, Stickin' Around, Ace Ventura, Free Willy, Feivel's American Tails and Donkey Kong Country
Disney has also used the company heavily since the 1980s. To put this into perspective: The only Disney animated series where Wang wasn't utilized as an overseas studio have been: The Wuzzles, Adventures Of The Gummi Bears, Pepper Ann, Doug, Recess, House of Mouse, Lloyd in Space, Mickey Mouse Works, The Proud Family, The Buzz on Maggie, Kim Possible, and some of their more recent stuffnote including Motorcity, Gravity Falls and Star vs. the Forces of Evil. Every other show has used them.