Breath of the Wild is a fairly unique experience, especially within the context of the greater Zelda franchise.
Mario Kart 8 Deluxe takes the same formula as previous Mario Kart games and refines it to perfection.
Both are excellent - I'd give the nod to BotW for both novelty and play time, but you'd be good with either.
Kill 1 man: You are a murderer. Kill 10 men: You are a monster.
Kill 100 men: You are a hero. Kill 10,000 men, you are a conqueror!