Mario Kart at about 10 hours. I've pretty much played it only online surprisingly, despite not usually being an online kind of guy, so this is also the most hours I've put into a game online. That's also why I don't plan on getting Arms or Splatoon, so it'll be a while before anything else I'm really interested in comes out (either Mario Odyssey or Kingdom Battle).
I think I have like 2 hours in Mario Kart. I have maybe an hour in Disgaea 5.
I have two friends, neither of which want to game when we're together in person. And I don't like the idea of playing online without a wired connection...
Rabbids Mario might top the other games though. It looks like a blast.
Mario Kart 8 Deluxe followed by Splatoon 2. I have tons of 10-25 hours played games as I tend to go from game to game quite rapidly. I'm currently sitting at 27 Switch games and only a few have been played for "a little while".