Across all three platforms? Pretty sure Modern Warfare 3 destroyed Skyrim with playtime... especially with the PS3 version being unplayable and all.
I'm sad now

EDIT: After reading it, it won most played RPG, and the competitors either svcked, or weren't out for very long either... the most played game WAS Modern Warfare 3 as far as total playtime goes. Skyrim only won longest sessions. Misleading title, hoooo!!! Also [censored] the PDFs take forever to load.