[Note: this is for Mario Kart Double Dash, not your fancy-schmancy Wii version. What am I, made of money?]
1. Baby Park
I don't think I've finished better than 3rd (150 speed) on this course in months. The sheer amount of shit clogging up the track makes for an incredible obstacle course, and it's basically an endurance contest of who can sustain the most damage and still win. "Baby" don't mean "pussy."
2. Yoshi Circuit
In terms of pure racing, it's the hardest course in the game. Also, there's that secret underpass that I always go for, foolishly, and end up in the drink. Because Yoshi is an asshole.
3. Donkey Kong Mountain
That cannon is a blast. A BLAST! Right?? Oh, shut up.
4. Peach Beach
Because that detour across the tiny island is Mario Kart the way it should be played. Recklessly and drawn to the pretty colors on the ramp.
5. Bowser's Castle
...Which is apparently so badass a course that it refuses to allow itself to be screencapped.
Oh, and the Worst Course in Mario Kart?
Fuck off and die, Dry Dry Desert.