For me, it would be Super Smash Bros. X (Brawl), hands down.
It's probably the only game of this year that I played since it came out (since it came out in Japan, no less) and that I'm still playing on an at least weekly basis six months later. In fact, I can't remember the last non-Pokémon game where I've done that (maybe GoldenEye?). Certainly not this generation, anyway.