Crikey, a game I'll still be playing in twenty years time? That's a tough one.
Still, I'll take a crack at it and say...Fate Core? While I haven't run anything with it for a year or so now, I have played the occasional game in that time, and I do intend to run it again sometime.
The system has enough flexibility and can be applied to a wide enough variety of genres that I think it has a good chance of enduring for another twenty years. Even if it's in a newer incarnation like Fate Core Plus or whatever variation is making the rounds in 2034.