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. 

In my experience, shoot'em-ups are easy to make, but hard to get right. Same with text adventures, only more so: while it's trivial to make a bad one, by the time you're done with testing, bug-fixing and polishing you'll have spent months -- longer than you would on some graphical games. Speaking of which: it's definitely easier to script a visual novel, but then you need to make or acquire all that graphics and audio.

At the other end of the spectrum, it took me several tries to even end up with a complete, playable roguelike. It's easy to get a little @ tromping around a grid bumping into walls; after that, difficulty spikes. And it's all programming -- no shortcuts to take! But it's very satisfying when it finally works.

I don't know about the hardest or easiest, but a hard one to mention would be Real Time Strategy.

Programming is not simple, because you have to manage AI and path-finding of sometimes thousands of instances. But the true challenge is balancing it for online play when you have to  try to account for every single possible combination of units and strategies to work against every other one. Especially if you use multiple playable teams that play uniquely different.

It's not an easy question to answer, but I shall try... to me the hardest one to make would be an MMO like someone up there said.

The easiest one to make hmmm... maybe a 2d puzzle platformer?