Don't make me chose between my babies!
Mario consistently made me smile the whole way through and has some astounding moments, but Zelda did genuinely make me feel like a child playing Zelda for the first time again. Also, I think Zelda had the more challenging and fulfilling gameplay throughout, with a somewhat weak ending. Odyssey isn't particular challenging for the most part (nor does it need to be, Mario games are meant to be accessible), but the entire last 4th of the game is amazing and I'm still in shock of how much I loved it.
If it were allowed, I'd say both games are my Game of the Year so far. They're both just that good.