If you count only single playthroughs, I believe it's Dragon Quest 8 with (IIRC) 107 hours. I'm pretty sure Zelda: BotW is going to overtake that though, as I have around 80 hours so far and still have many sections of the map to explore and the main quest I'm guessing 3/4 or so done, not to mention all of the in-depth exploration I can still do in the areas I know.
If counting replays, probably Mario 64 is tops - I'm guessing I've put around 400+ hours into it because of all of the times I've replayed it, mostly speedrunning, and I still play it today from time to time. I might even be selling that time short. NHL 2000 is up there too as I've played a lot of full seasons on it (going to say 200+ hours?), and FF7 I've played through 3 times and will probably do a 4th someday, for a total of maybe 150 hours.
If you want to be cute and call World of Warcraft soloing to be single player, than that would take the cake though. I never spent all that much time playing with others (though it's still a decent chunk), but I'm guessing I have around 20-25 days played over all characters I've ever made without ever making it to max level. Done with that game though, can't get into it without being bored now.