I really enjoy the classic characters. My go-to-guy would be Yoshi because of the Mario games that I have used Yoshi in. Yoshi is just that character you never get bored of using. I don't know, I just never get bored of him.
My guy has always been Mario. I like to keep it classic so Mario will always be by number 1 choice, that is whenever my friends haven already stolen him. I really like his fire ball power up and his combination of kart stats is a nice plus.
I'm always Mario as well, unless I'm playing with a large group of friends, in that case I'm Bowser and I don't care about winning, my goal is just to knock as many people as possible into traps or off the course.