Answer by Martin Fox:
We're not counting bridges, right?
I think it may be the Busan Cinema Center in Busan, South Korea. It has 163 meter long roof, with an 85 meter cantilever portion. The Center was designed by Coop Himmelb(l)au and opened in 2011.
Second place may go to the Marina Sands in Singapore, built in 2010.
The Marina Sands was designed by Mose Safdie and engineered by Arup.
The "Skypark" on top of the building includes a 213 foot (63 meter) cantilever, at 200 meters height.
It's an impressive building for sure, although I can't help but feel that it looks like a cruise ship marooned on top of skyscrapers.