Breath of the Wild, because I didn't have a Switch yet. It's the only game I ever bought twice.
I will probably buy it for the Switch as well when I get one next year. I have little time now, but when I do play my Wii U it is Zelda BotW. I would love to play it on my lunch breaks at work instead of looking at work related reports and other boring stuff. Really glad they put effort into the Wii U version.