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.
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.