I usually make levels to help me get better at playing Mario games, so they're all difficult. The easiest is going to be Spin Class (36E8-0000-018B-A06D). The hardest is Breakneck Castle (68C9-0000-019C-F084). The newest is Deconstruction (C722-0000-01AF-A08B).
I also got tired of doing the crazy hard levels, so there are a couple of silly easy ones: "Goomba's Day Off" and "Exciting Cave!" I don't have the codes on me, but you find them if you follow one of the others to my profile-ish thing.
Mario Maker Level Thread
haloopdyCreated:
I forget the name of the level, but here's the key: 56FC-OOOO-O213-627F I've made other levels on that account, so try 'em out!
My friends made some of the levels too... (bombs away, and ship levels are a few examples)