I'll go for Mario Kart 8. It's the best game in a series of games of which none of really been bad.
Between the courses, the fantastic visuals, antigrav's subtle influence, item balance etc. the game hardly made a single fault when it came to racing. While the Battle Mode did get mangled in the process, its never been my favourite mode and the game makes up for it in DLC. For £11, the game suddenly becomes at least 50% larger than every other Mario Kart ever, with fanservicey courses and the dreams of "Super Smash Kart" featuring several Nintendo series finally starting to come true. It's a flawless execution of everything I like, and everything I ever wanted from Mario Kart.
The main other contender here should be Smash Bros, but I've only owned that for 7 days compared to 7 months with MK8. It's another fantastic addition to the Smash series, but I'm constantly conflicted by the competitive and non-competitive side, and even the completionist dimensions of Smash Bros.
Little things keep popping up preventing me from fully focusing on any of the above strategies. Diddy Kong is teetering on the edge of being unbalanced, certain items like Gust Bellows are ridiculously overpowered even in a casual match, while a whole host of random item drops and tedious challenges make completing the game more of a chore than a challenge.
Don't get me wrong; This is a brilliant game, but it feels like in pleasing everybody they only went 95% of the way in each direction with that little slip in the level of attention awarded to each style of playing giving way to imperfections slipping through into the final game.