Went through some of the custom songs today. There are some neat ideas and definitely some cool patterns, for sure, but the community on average don't make the best beat maps.
I'm gonna go through some more later, but for now, very high recommendation for the Imagine Dragons song, Believer. The chart is fantastic, in line with other expert-difficult songs, and is a blast to play.
Modding is hyper easy, BTW. Extract a zip in a folder and drop one .exe onto another. That's it!
Edit: After doing it again, Believer is easily the best beat chart in the game, exceeding any of the included songs. It's incredible. DO IT.