Skyrim's up there, with roughly 150 hours. Oblivion may be ahead of it, since I have played multiple files.
I maxed out the game clock in Jump Ultimate Stars, so I have no idea how long I've played that for, but it's at least 100 hours.
Finally, Kingdom Hearts II. I've beaten the game maybe four or five times, and that takes about 40-50 hours. Now, we add in my extended save file, which I've brought up to about 90 hours, and I'm not done with it yet. So that's what, ~300 hours? Not counting any potential save files where I hadn't finished.
Now, if we have an element of synonymity between 'games' and 'series of games,' then the order, from most to least hours, goes Kingdom Hearts by a ridiculously long shot, with Pokémon and Elder Scrolls locked in a heated battle for second place because I can't remember all the past save files I've had.