If I had to choose one game to play for next several years exclusively, it would have to be something likely competitive and online.
I say competitive, because no matter how good you get at a game, there's a significant chance that someone will best you. There will almost always be someone new reaching for the top.
So if I was forced into only one game for, let's say for the sake of this example, 15 years, it would have to be something competitive, and something that required skill that can constantly be improved upon in my own body and reflexes, rather than strictly relying on the games abstract mechanics.
So far, I've boiled my choice down to online shooters. There's always a large pool of players, most of which play completely different from each other, and it's a game based mostly off of player skill rather than strictly relying on mechanics (though I realize there are elements of shooters that can be exploited).
Now I must distill my choice from this into one particular shooter. This is a little tough, but I'm sure it can be done.
Let's look at the most often played games on Steam (I will not be including survival games in this, just as an arbitrary stipulation). Currently, the top 5 most played competitive shooters are, in order:
1. Counter-Strike GO
2. Team Fortress 2
3. Garry's Mod (I will count this only because there are competitive modes, which might be in it's favor for this decision)
4. GTA 5
5. ARMA 3
Looking at player counts, it's clear that Counter Strike has been steadily increasing in player size just within the last two years, and remain pretty consistent.
Team Fortress 2 also has large and fairly consistent player count.
Garry's Mod has had a few large gaps in player counts dropping, but remains overall high.
GTA 5 peaked quite high, but has steadily been decreasing since release, with a rather small install base.
ARMA 3 has been consistent, but usually quite low.
This being stated, I'm picking between Counter Strike and Garry's mod.
Counter Strike does have the benefit of custom maps, weapons, and a decent learning curve for it's weapons, but overall uses the same mechanics. Garry's Mod on the other hand, while largely keeping the same FPS gameplay from the Half Life series, does have a robust amount of different game modes, so gameplay can be mixed up and kept fresh. Unfortunately, with so many game modes, the player base is divided quite a bit into each of these modes, meaning there will likely not be a very continuous player count for any one mode.
However, I do pick Garry's Mod. The prospect of spending so much time with Counter Strike with so many different people, and getting progressively better as I play is enticing, but I much more fancy the idea of having a more varied number of gameplay modes and mechanics in Garry's Mod, not to mention the additional content I could potentially create myself.
TL;DR I'm stupid and did more research than my answer was worth. I picked Garry's Mod.