Oh shiznit, I was so thinking of starting a poll about this. Might still do that, but there's only 10 choices and more than 10 maps, although I think some are more popular generally than others.
Wario's Stadium is definately a good one. It's a big endurance test and there's ample room for powerslides. And the shortcut at the beginning gives it an interesting element, even though it is kinda cheating.
Personally, one of my favorites is the Koopa Beach. It's short, simple, interesting, and there's a good amount of variety in it for it's size. Also, there's two short cuts, but they're not enough to put people too incredibly far ahead of the rest. They're just right.
I think the shortcut through the waterfall gives you an extreme advantage.. I used to tell people who were new to the game to cut through the waterfall (backward) as a short cut.. and they'd get way behind.
Man, I used to play this game with my parents.. completely forgot about those days.. they were horrible at it.