It took me all day and this is where I landed. Both of these games are on my Mt Rushmore so this was the hardest decision. SMW is one of my most replayed, favorite childhood games. MK8 is my favorite game of my adult life, and is the reason I met this group. A few pros for MK8:
-best multiplayer game ever for me
-met the best group of online gaming friends through it, and it's a continual bonding experience with that group
-I have a history of playing Kart with family
-the multiplayer aspect gives a level of satisfaction for victories that single player games don't give
And for SMW:
-it was a gamechanger, and a masterclass in platforming
-nostalgia. I will enjoy replaying this the rest of my life. I may not say the same for MK8, after its sequels are released
-inspired so many games
-timeless

If I had to pick the Mario Kart series versus Mario, I would probably go with Mario Kart. That's had a more meaningful impact to me overall. But for an individual game, I somewhat begrudgingly pick SMW.

Also @darthsmeagol offered me a wheelbarrow full of Loonies for this