Okay, let's revive this old thread and hope some more players are giving their opinions — personally, I always find it interesting how different people think about this.
To not only post nonsense here just to push the thread up, I had a project for the last two days: I tried to win a 3-lap-race on
all tracks (also including add-on tracks, but excluding The Old Island (thanks to daBascht

) and What A Mesh (sorry, A.

) with 9 enemy AI karts, so I would see how hard the tracks really are.
Here are my results, extended by my personal opinion:
Easiest Tracks1.
Star Track (just watch the AI dropping down and pass by ...)
2.
Math Class (as long as you hit the zippers, it should be no problem)
3.
Shifting Sands (yes, what to say? Just drive ...)
4.
Coyote Canyon (the same ...)
5.
Hacienda (hurray! collecting nitro for free!)
Hardest Tracks0.
The Old Island /
What A Mesh (don't count here

For these, I'm too bad. )
1.
Lighthouse ("wohooo... ou, there I was *not* supposed to drive along.")
2.
The Island (easier than the old version, but still quite hard)
3.
Old Mine (there's nothing to explain, is it?)
4.
Zen Garden (can be really challenging with many AIs)
5.
Secret Garden (IMO a bit easier than Zen Garden, but still not easy if many AIs are present)
Funniest Tracks1.
The Cube (Goin' 'round in circles...)
2.
Math Class (high-speed action!)
3.
XR591 (fast, many turns, awesome once you know how to drive)
4.
Secret Garden (driving through the plants, not seeing anything ... yeah, poor Zen Garden, you didn't convince me yet totally

)
5.
Fort Magma (hey, I'm driving Nolok, what do you expect?)
Most Annoying1.
What A Mesh (sorry again

)
2.
Classic Volcano (aaargh... that downhill part -.- )
3.
Star Track (zzz....zzZ.... Zzz... boring)
4.
Doing the Dishes (boing, boing, boing — where is the "road"?)
5.
The Cube (this one IS funny, but still ... I always become dizzy.

)
Best Looking1.
On an Iceberg (wow...)
2.
Zen Garden (wow again...)
3.
Secret Garden (still looking great)
4.
Old Mine (those new moving karts .. those lamps ... really a nice track)
5.
Scotland (I really like how that track looks — if only the bonuses were placed better ...)
(I did not list Rudy's new SubSea track here, because it's not ready yet, but if I had to rank it in its *current* state, it already would be > Secret Garden.)Worst Looking1.
What A Mesh (and again sorry

)
2.
Horizon (hey, are those thingys *buildings*? And is that supposed to be a *zeppelin*?!)
3.
Classic Volcano (okay, it's quite old, but I'll mention it nevertheless ...)
4.
Classic Subsea (... as I mention this one ...)
5.
Classic Tollway (... and this one, but this one looks okay!)
Generally Favorites1.
Math Class (high-speed action *repeat*)
2.
The Cube (aaand round and round and round and round and... )
3.
Fort Magma (for relaxed driving ... Nolok likes his home)
4.
On an Iceberg (yay! Great looking, nice to drive — if there would not be this one undrivable turn...)
5.
Old Mine (yohoooo! oh, shit, crashed.)
Least Favorites1.
The Old Island (urghs.)
2.
Classic Volcano (urghs again.)
3.
The Island (I don't like this narrow street... neither in the old version, nor in the new one)
4.
Star Track (what? Still not finished? Looks the same all track long... only the graphics outside the track are really nice, too bad that the track itself is so tiring

)
5.
Horizon (hm. ugly. not very funny.)