According to Eurogamer, Mario + Rabbids Kingdom Battle is currently the highest selling 3rd party game for the Switch.
Only catch is digital sales are not included, not all regions/markets were included for sales, and sales for other big 3rd party games like Minecraft are not publicly known.
http://www.eurogamer.net/articles/20...-nintendo-game
https://www.destructoid.com/mario-ra...e-462525.phtml