Well, I haven't looked at every game that was released this year, but it's undeniable that Nintendo knocked it out of the park with Mario Kart 8.
The game is consequently beautiful, the next course even better than the last. Everything about the design seems to be perfectly in tune. The amount of care they took in all the little animations, characters and backgrounds (and what goes on in those backgrounds) really shows the love and attention the developers gave this game. This game shines in it's art-direction. I don't know if it was 'the prettiest' of the year, but it surely is a contender.
And to add my two cents to the whole 'realism' debate;
You have realistic looking games and you have 'realistic' looking games. There's games that try too hard to be like reality. Racing games are the most obvious offenders. To me, most look 'boring' or uninspired, because they look the same as when I look outside my window in real-life, but worse. They also almost always look exactly the same amongst each-other. I don't care about those couple extra particles here and there, that isn't what gives a warm feeling inside.
To insinuate that I or anyone therefore doesn't like 'realistic' games is stupid, because Bioshock also looks realistic. Those kind of games however have a certain 'flair' to them. I want to see something that's unique and recognizable, that has features which separate it from the masses. It must have an obvious direction where an art-director took clear decisions instead of taking the 'easy', straight-forward route and make it it's own product. In that regard, Mario Kart 8 to me undeniably wipes the floor with Drive Club. Art and design, they are key. That's what makes a game interesting to look at to me, what gives me that warm feeling.