I've only got it on GameCube.
From what I remember, I once did a lap of Baby Park in 9.5 seconds.
( iEC: Redux Hands up., Fri 29 Dec 2006, 16:45, archived)

I think my fastest is about the same, but it's possible you've got me beat there
However, Baby Park isn't the best test of skill. It's just an oval. DK Mountain, that's a test of skill.
( Dave the Mighty, Fri 29 Dec 2006, 16:50, archived)

DK Mountain is fucking awesome.
I'd say Rainbow Road is the best test of handling, and Wario Colosseum is the best test of item use.
( iEC: Redux Hands up., Fri 29 Dec 2006, 16:52, archived)