For my own levels, I agree that Dark Sea 1 is a good start. Amidst the Clouds too, perhaps. Industrial Trap is really close in difficulty, as its obstacles are generally long and complicated, so it's probably not a good jumping off point. And the rest of my levels aren't worth suggesting.
As for other levels, you can just search for Very Hard levels on the forum that get a lot of attention. Generally those will be worth the time.
I must also note that you just have to do the best of your own abilities, and don't stress out if you can't legitimately beat my level as a whole before the end of the voting period. It's very long and very challenging and it takes a quite some time to beat for almost anybody.