There's 32 courses on Mario Kart Wii. But i dont think I can pick a fave. These are my opinions on each of them

Mushroom Cup
Luigi Circuit-It's a standard Circuit found in every game. It's a bit boring but good thing is that its not a difficult track to master
Moo Moo Meadows-It's not too bad its pretty basic but a bit more fun than Luigi Circuit and the hazards are easy to avoid.
Mushroom Gorge-It's really fun to bounce on the mushrooms but just make sure you stay in control so you dont fall of but i find it harder to fall off than stay on
Toad's Factory-Probably the first circuit that can be a real challenge. You have to dodge boxes and its hard to drive in the mud but its still a good track.

Shell Cup
GCN Peach Beach-One of my favorite tracks from Double Dash and it's a really easy and fun track but I don't like those walking things that block you.
DS Yoshi Falls-It's a pretty quick track and basic; just going around in circles. If you struggle here you're a total fail.
SNES Ghost Valley 2-I don't really like the SNES tracks but its not really hard but just keep in mind that there's a lot of turning for those of you who struggle in that area
N64 Mario Raceway-I like this track its lengthy, fun, and just great. I don't know what it is about it but its just really enjoyable.

Flower Cup
Mario Circuit-A standard Circuit found in every game it seems. It's really hard to struggle in this course.
Coconut Mall-It's a very fun course with escalators, fountains, and cars and stuff its perfect for people that are banned from their local malls for skating in them or something lol.
DK Summit-It's just an epic course going down a snowy mountain and going through lots of speed boosts and stuff its a great course but remember to keep control.
Wario's Gold Mine-It's neither hard nor quite easy. In many portions of the race its a narrow track which you have to share with other racers, trains carriying goal, and bats so falling off will likely happen once or twice unless ur an expert at this game.

Banana Cup
N64 Sherbet Land-Lots of room to work in, it may be a bit harder to control ur player on the ice but overall its not too bad as long as you can avoid the penguins in the more narrow portions of the race.
GBA Shy Guy Beach-I'm not a fan of this race its easy to bump into stuff and the bombs have a big range but you can get the hang of it easily.
DS Delfino Square-I know Mario Kart DS fans like this track. It's a pretty easy track without road hazards and you'll find it to be quite enjoyable.
GCN Waluigi Stadium-Not as fun as DK Summit but its still great for tricks and stuff but make sure you can control yourself or you'll bump into everything.

Star Cup
Daisy Circuit-Wow Daisy finally gets a circuit. It sort of reminds me of Delfino Square and its a fun course that you'll like whether your a beignner or expert
Koopa Cape-Like Daisy Circuit this course is not difficult to master but its a bit more excited with all the water and the tricks and the electricty and stuff
Maple Tree Way-I like this track its just so fun for some reason and if u run into the leaf piles in a certain way you can get free items
Grumble Volcano-Known across the internet for their shortcuts, this is a very fun circuit. At first you'll find urself struggling with the obstacles but you'll get the hang of it easily.

Leaf Cup
DS Desert Hills-Just your basic desert stage. It's pretty bland and it's not my favorite track and it's not really that hard either.
GBA Bowser Castle 3-You gotta have excellent control and be focused the whole time otherwise this track will eat you alive.
N64 DK's Jungle Parkway-Not even close to being my favorite DK stage in the game but its still not too bad. I enjoy it but i find it very difficult to recover if i'm off track even once.
GCN Mario Circuit-Not really that different from the Mario Circuit in the Flower cup but by know a circuit like this should trouble you too much.

Special Cup
Dry Dry Ruins-Definitely the best desert race and the more challenging one as well. There's a few obstacles to avoid but definitely a good track.
Moonview Highway-It's a pretty exciting track with a few shortcuts i believe but if you suck at avoiding obstacles this is gonna be a nightmare for you.
Bowser's Castle-Nothing you wouldn't expect from any other Bowser Castle tracks. I didn't see myself having too much of a hard time.
Rainbow Road-Just like the other Rainbow Roads. Expect to fall of several times when you're just getting the hang of things but its still a really cool track.

Lightning Cup
SNES Mario Circuit 3-Really basic and not fun compared to the other tracks.
DS Peach Gardens-Another great Peach track since there's a lot of turning and control involved which is a good challenge for the more experienced players.
GCN DK Mountain-If you mastered DK Summit then you'll do well in this track. There's a few obstacles that could give you trouble but none of them are that hard to avoid.
N64 Bowser's Castle-Um Idk what to say that I haven't said about other Bowser Castle tracks. It's fun I guess.
So what are your favorites?

I'd have to say that my overall favorite stage is Maple Treeway. I just love the way it twists and turns, and I enjoy stages with cannons, since they let you avoid those dreaded Spiny Blue Shells. My favorite part of the course is the u-turn section with the trick ramp. I enjoy speeding up twice on the boost panels, then grabbing an item mid-trick. That one area often helps me get ahead a couple of positions, and I just like the feel of the curve. Other than that, some other stages I enjoy are Koopa Cape, Sherbert Land, and DK Summit. Some stages I just positively hate are Moonview Highway, Grumble Volcano, Ghost Valley 2, and Wario's Goldmine. I must just have really bad luck in those places or something.

I love playing on Rainbow road nd having my friends fall while I get to go ahead. And when I play alone, I like to play on DK Summit and Coconut Mall.
On coin battle I like to play on the one with the big ramps and for balloon battle, well the one with the chain chomp.

Favourties: GCN DK Mountain, N64 Bowser's castle, SNES Mario Circuit 3, DS Delfino Square

I didn't like Mario Kart Wii. Besides the expected complaints about winning or losing depending entirely on items, the whole thing felt like a step backwards from Double Dash!! and DS and a number of... things... in it... felt like insults to the Mario fiction (mostly I just have a beef with Miis showing up in games where they don't belong). I gave it away and have not missed it.