Over 100 hours:
Probably Ocarina of Time, me and my friend beat that game around 30 times.
Pokémon Blue, was my whole life until Silver came out.
Pokémon Silver, I managed to complete the Pokedex in this one.
Smash Bros. Brawl, I know that this isn't thought of as a single player game but I held European records for the Home-Run Contest and did that all available time for a while.
Mirror's Edge, I used to speed run Mirror's Edge time trials.
Trials HD, all platinum medals and built levels, was apart of the community for this game for a while.
Mass Effect, think I've played this through about 20 times.
Fallout 3, 150 hours on one save, got all achievements on the Xbox 360 version.
Dark Souls, over 50 hours on console and 73 hours on Steam. One of the best games of all time.
Witcher 3, 130 hours on one save and still have one main quest to go. Then DLC after that, but I don't know when I'll get back to it!
Dark Souls 3, 96 hours and still have to play DLC.