I'm above the [Skilled] rank, so I tend to farm maps higher than the Hard difficulty. Here are some of my favorites!
Easy maps: Creeper, LuckyBlock
Medium maps: MushroomLand, Rythm
Hard maps: Magical, Pipe ( note: a lot of people think Pipe is really hard, but once you get the flow of things, it's quite easy. )
Expert maps: ShatteredWorld, ZenaniTower
Insane maps: Castle ( with an e-pearl )
Out of these, ShatteredWorld is definitely my favored map. It's quite long, but it's easy, and one of the few maps I can do over and over without getting annoyed, haha.