For me it was Tetris as a child
By far its Fire pro returns I have had it since it came out on ps2 and have put ridiculous amounts of time creating edits and simming matches.First and foremost, there's NO WAY I can know how many hours I've spent on games like SMW, SMB3, Street Fighter 2, Ocarina of Time, Banjo-Kazooie, the PS2 era PES games, Smackdown vs. Raw 2006, Smackdown Here Comes the Pain, Sonic 1 and 2, Mega Man 2, Sim City, etc. all my life. Back then it was very rare for a game to keep track of the time you spent in it.
The platforms that have the most detailed info about this that I have are the Nintendo 3DS and Steam, so afaik, these are the games that, as of today, I've put the most hours into in my life:
1. Football Manager 2011: 16 days and 51 minutes. Which means about 384-385 hours. FM games are big time sinkers, man. And this isn't that impressive considering most of the biggest FM fans I know seem to spend 500-600 hours per game on average.
2. Animal Crossing: New Leaf: 289 hours and 11 minutes. This game is almost like a second life. So much to do and things happen in real time and the weather and other things change according to the season. You don't really notice the time going by as you play this game.
3. Sid Meier's Civilization V: 190 hours. And I've only seriously played this one for like 2-3 months. Fucking Civ, man. JUST ONE MORE TURN.
4. Ultra Street Fighter IV: 159 hours. This was the first fighting game that I played online and the one I played the most online by far. Hence why 159 hours.
5. Super Smash Bros. Melee: I forgot exactly how many hours I spent on this one and don't feel like pulling out the GC to check it now, but I definitely went over 100 and less than 150. I'm gonna say between 108 and 113 hours.
Adventure Capitalist: 103 hours... yes, I spent 103 hours on a clicker game.
Pokémon Omega Ruby: 95 hours and 14 minutes. So many Pokémon to catch here and plus there's a bit of a post game and you gotta beat the Elite Four twice. No, I don't hunt for shinies nor do I play online.
Dragon Quest VIII: About 91 hours iirc. This game is massive. Bigger than it looks.
What about you?