I've always been either a Mario man or a fan of the bad boy Wario. On any Nintendo game I play with them in, it's a rare day where you won't see me using them to do whatever it is I'm doing. I've always loved Wario because of that mad moustache game he's got going and his dinky little yellow hat. It just screams to me for some reason - I'm unsure who I am when I look at Wario. Ok, maybe took that one a bit too far but yeah. Wario or Mario anyday for me!
This depends on what we're playing. Mario games, it's Mario for obvious reason, for Mario Kart it's Bowser, he has a slow acceleration, but his top speed is amazing and a couple drifts will get you there. For smash, if it has to be a strict Nintendo character, it's Link or Samus. Overall my favorite Nintendo character is Link as the Zelda series, minus Breath of the Wild, are my favorite series that Nintendo has out.
Back before you had a choice of many characters for the various Nintendo games, I actually liked Luigi since green is my favorite color. Now, I often find myself using Link or Bowser, depending on the game.