My favourite part is actually overworld design. I love adding hidden goodies here and there, and I also love working with layers.
Dungeon design is somewhat tedious for me, mostly since I use the default Zelda NES style, which right off the bat doesn't give me a lot to do in terms of complex puzzle design. Also, I usually just make short and simple dungeons, since I'm lazy and don't know what to do with the tools given to me and most of my dungeons end up being screen after screen of block formations and water.