Mario Kart for sure.
One I play for fun, the other is both fun but serious for me.

They're both fun and serious to me. You can't play either one without getting competitive IMO.

I don't like like Mario Kart, but smash bros is one of my favorite series. I really see how you can compare them. As far as which is better, melee is arguably Nintendo's greatest creation. Game is still one of the top most played competitive games after 12 years. That games legacy is pretty amazing. Mario kart I would say is something more comparable to Mario party.

Ok so I get competitive with Smash too, I just know I'm terrible at it. Also I put like zero time into learning any strategies for it unlike Kart.

Super Smash Bros. That is all.

Same with me. It's a lot more fun playing Smash when you have to think of what to do on the spot. With Mario Kart, I like to be prepared.

This is a tough and possibly a question I might never be able to answer.  

When I'm in the mood for speed it will always be Mario Kart.  There simply isn't another racing game out there that even compares or comes close to the fun and excitement this game has to offer.  To show you how much I love the game, I'm forming this opinion based off of jsut the standard Wii system game and not even the Wii U version.  I have seen it played but have not yet played myself.  (will get to it)........

Smash Bros. is the kind of game I steer towards when I want to beat the crap out of something and laugh while doing it.  I love how they bring all of these different characters together for epic battles.  I love the different modes and it has such a variety on how to play different variations of the same game that I didn't get bored of the game ever.  

So to sum it up I believe both are the best  

I hope this hasn't confused you as much as me as I am torn to pieces after this kind of question lol.

Never played either but SSB looks fun and I've been actively researching the game lately.

Both are great fun. One is a racer and the other is a party fighter. Hard to compare. Had amazing experiences as a child for both.

Mario Kart no matter the version is by far the best racing game out there.