There aren't any 'easy' courses.
I did mine on Bowser's castle, Rainbow Road, Maple Treeway and Koopa Cape.
They aren't the easiest ones on the game, but they are the tracks I like the most, so I was naturally good at them.
Just pick the tracks that your good at. A track you might find really easy, someone else might find really hard.
And try all of them. I have unlocked all the expert ghosts, it's not as hard as it seems.