Most challenging game you have ever played

[Paatar]

For me it is Shin Megami Tensei: Devil Survivor Overclocked on the 3DS..

I just beat it last night (took me five hours to beat the whole final boss sequence) and holy fudgcicles. This game truely is challenging, and I am familiar with the Shin Megami Tensei franchise. So for it to be pretty challenging for someone who is used to how these games work, I can't imagine it for someone who starts out the series with this game.

It's a fantastic game that has tons of replay ability and the story is engaging. But I do not think I have ever gotten as frustrated at a game than I did with this.