Double Dash was just the most fun for me. Even though there were only 4 cups. The ones like MKDS or MK7 were lotsa fun but DD left the biggest impression for me.
I want to say Mario Kart Wii because it's so much fun, but I have to go with Mario Kart 64. It was my frist Mario Kart game and I instantly fell in love with it.
I'm gonna give Mario Kart 7 the slight edge over double dash, just because of online play and more cups. I'd love for them to go back to the two characters per kart play style of Double Dash. Not only did that add another element to the races, but it was just fun having that many characters.