The hardest game that I've played... THat's actually a hard one because I like all games, even if I rage!
To quote serge, I thought that Halo 2 Campaign was hard on vetern. THose jackals basically did aimbot, but, if you knew where they were and were quick enough, you could take them out with no issue. And, with me, I used glitches to get further in the first few levels. The second level was the most famous for me using glitches, since I could bypass the hunters at the gate, and also get more sniper ammo when you pass the Pelican the floats off. And past the bugs/other jackals.
But, other hard ones that I've played, EDF Inferno solo. It's hard because I don't have that much of good weapons. Although, I did farm the first boss with my frind and got the best sniper in the game that I can think of. 2700 damage per shot. I think like 6-7 shots per clip, and speed was decent.