Mine is actually Super Bomberman R still, but MK8 and Arms are both growing much faster than it at this point, and Splatoon will overtake them all within a month of it being out probably (cept BOTW natch)
To my surprise, it has been Has Been Heroes by a huge margin (other than BOTW). I say to my surprise because when I first started playing it, it hadn't "clicked" with me and I thought I didn't like it. I came back to it, and it all made sense and I found myself "in the groove" with the gameplay and lane management. It's now one of my favorite Switch games. I have put in over 60 hours into it!
Nothing at the moment. No other retail releases are jumping out to me right now, but I'm considering Splatoon 2. There's a digital game or two that i might grab at a lower price, but I'm not desperate for anything on the Switch at the moment, got enough to play on other systems.
Mario Kart and that will probably be the case until Mario Odyssey comes out. I play the heck out of ARMS and Splatoon come Friday, but I keep going back to Mario Kart for a couple races every other day or so
Up until recently it was MK8D, but I have put a lot of time into Isaac this week to reclaim the second spot. Plan on purchasing Mighty Gunvolt Burst and Implosion soon so I will finally have something to call a "library" on my Switch.