MMORPGs are definitely the toughest to make. AAA developers struggle with this and often fail, not to mention the poor indies. If you, you who is reading this now, are an indie dev making an MMO, I tip my hat to you. Anyway, as soon as you introduce online multiplayer for 2 players to your game, the production gets increasingly more difficult. And then add the prefix "massive".
The easiest games are naturally the simplest. But in the end it all depends on the scope of your project.