For most solo player games, easy. For most multiplayer (co-op) games, the most challenging option. For me, I am more usually interested in the mechanics and story of the solo player game than the challenge. For Co-op games, I value the exploration and strategic planning, struggling with friends. I don't feel any reward for being amazing in a single player rpg type game after hours of struggling, and in fact I feel pretty sad about it... but on the other hand, for co-op I'm usually lost in the moment in a difficult situation. Because of the momentum, the frustration often doesn't turn into boredom for co-op games, like single player games of constant endless hours at a time, retries.
This question reminded me of when Left4Dead first game out. I grabbed my 3 closest friends, and made sure no one even TRIED the game before we all get started at the same time. It was the long weekend. We started off with the highest difficulty level, without any knowledge of how to do anything and struggled through. It was amazing. Probably because it had horror element in it and we weren't overrun with the knowledge of what's going to happen before it did, and had to strategize for the unknown while splitting resources and backing each other up.