It's impossible to answer.
Too easy can be ok if it's presented in an interesting and engaging way: i.e. Assassin's Creed 2
Conversely, hard is not always a good thing if it's artificial. Being artificially hard is games like S.T.A.L.K.E.R. where the enemy has perfect aim, can see in the dark, and always out number you. Basically any game where the difficulty is the result of poor mechanics or when the Unfair advantages just for the sake of difficulty.
Too easy can be ok if it's presented in an interesting and engaging way: i.e. Assassin's Creed 2
Conversely, hard is not always a good thing if it's artificial. Being artificially hard is games like S.T.A.L.K.E.R. where the enemy has perfect aim, can see in the dark, and always out number you. Basically any game where the difficulty is the result of poor mechanics or when the Unfair advantages just for the sake of difficulty.