I'm going to go with Mario Kart 8, as well!
I also feel there's problems with it, but it's still easily my GOTY for 2014. I just spent so much time on it and it's the most fun I had on a game last year. Nintendo really put a lot of effort in it, and for me it's the best Mario Kart ever made. If they add battle courses and tweak the weapons a little then it'd be pretty much perfect, though I wouldn't mind some character DLC, too.
Runners up would be Hyrule Warriors and Theatrhythm: Final Fantasy Curtain Call. I didn't really explore Hyrule Warriors fully and just played through the main quest mode, but I had a lot of fun with it and I think it was great to be able to play as so many of my favourite Zelda characters instead of just Link. Theatrhythm Curtain Call is just fantastic! One of the best music/rhythm games I've played and it's helped with discovering lots of new tracks from the Final Fantasy series that I really love. Nothing to really fault with it, it even has a nice catalog of DLC, with more on the way!
I'm actually going to say Smash Bros for the 3DS as a runner up, as well. I don't think its the best Smash Bros, but I'm still really impressed how they managed to get that game onto the 3DS looking so nice and running at 60fps. It doesn't feel like they cut back on too much either, it has most of the modes there and still tonnes of content to unlock via the challenges and trophies. It even had Smash Run which was superior to the Wii U's exclusive board game mode. I'm more impressed with this version than the Wii U version by a lot.
( Edited 15.01.2015 10:09 by Marzy )