I don't know the exact number, but for a single game, probably Runescape. I think I spent nearly a year on that game with friends when I was in middle school, so I likely had upwards of 1000 hours on that game.
I've also spent an ungodly amount of time playing the main Pokemon games. In particular, the 6th gen had me hooked for 1000+ hours (650 hours in Y, 350 hours in AS, and 100+ hours on the other two games that gen). I've overall probably spent close to 2000 hours on the main games as a whole, though I'm fully abandoning ship with Sw/Sh so I don't see that number going up anytime soon.