It's pretty simple. Why? Because the title says it all. What were the hardest games to beat that you ever played on iOS? 1. Tap Maze 2. Organ trail (on hard mode)
Modern Combat 3. The level where you were pinned to cover by a sniper used by the guy you were trying to kill. EDIT: Thought this was iPod games only. If it's not, then definitely CoD4 Mile High Club
^LOL. Seriously though on iOS, for me it would have to be 777 Deaths by Square Enix. In this game there are tons of mini-games in forms of Platformers, vertical runners with tilt controls, some are side-scrolling runners, none of them endless, all of them BRUTAL. And it's so good you cant stop hitting the restart button everytime you die.
Demons Souls...some of the bosses were ridiculous. Dark Souls was hard too but easier to summon help from other players...