You mean heaviest aircraft to land without power? That'd be the Air Canada 767 that Jubilee777 refers to.
If it's the heaviest craft to glide for a distance, period, it'd be the 747 that ingested the volcanic ash over Indonesia that BC. responds with. The 747 crew eventually successfully restarted all 4 engines, though, so it didn't glide to a landing...every question has complications! :-)