I don't own a Wii U neither a copy of SMM, but I'm legittimately interested in how people use the components in the game to create new mechanics/ themed levels/ challenging levels.

So far I have to say that the most challenging levels I've seen are those made by Ross, in particular "Fast" and "Companion Spring" as well as the famous "Winners don't do shrooms" by Kocobé.

And you ? what level do you think is the hardest ?

The hardest one is definitely Super Meat Bros! I almost destroyed my GamePad while trying to finish that level..

I agree that some of the above mentioned are tough, but they're certainly beatable. I ran into a few disgusting levels last night though, I wish I remembered the names...