Thats a difficult question.
Would the least frustrating game not be the easiest?
Personally I like a little (moderate) frustration, because it makes me want to become better.
(As Yahtzee said: "This shit will not beat me")
If a game does not frustrate me somewhat, I do not feel challenged.
However, the game that was the most fun for me without a real challenge must have been Plants versus Zombies, just because of its charm and humor.
(However, if you want to play that one hard, try creating a build that will survive forever in "Zombies, endless" ;-))
Apart from that, well, it is not all that hard to challenge me, I´m very far from a "pro" ;-)
Also, I used to play WoW like that, when I soloed. I would just breeze through the countryside, as it where, without really having to do anything. I don´t really know why I did not get incredably bored really soon. Paradoxically, raiding can be the most frustrating thing in the world, when 24 people do their best to pull their weight, and one jumps around standing in the "whatever does the most harm to the entire raid". (Those being the reasons I quit)