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?
I was thinking a puzzler or similar for easiest as well just because generally you know straight off the bat that you don't need to be realistic and that definitely helps. Especially if you pick like a game targeted for little children, "Add the number!", you know, that's obviously easy. But in terms of real games targeted to general audiences you may be right with the puzzle platformer!