Sebastian Lague is my favorite Unity tutorial creator.
Sebastian has a lot of really high quality tutorials on Youtube and this is his longest tutorial by far. Spread out over 25 parts. But you don't have to follow the entire tutorial to get something out of it - most of the concepts can be done and implemented in your own game regardless of the rest of the tutorial. The map editor is part 8 through 13 and is even built in its own scene so there's no need to do any of the other parts if that's the only thing you're interested it. In my opinion it is the best and most unique part of the tutorial.
All of his tutorials are available on Sebastian's Youtube channel, here:
The source code for all of his projects, including all of his tutorials, is available from Sebastian's Github, here: