curlycrouton said:
Operation Flashpoint, methinks.
I spent a LONG time beating that game. I still haven't finished the Soviet campaign yet, but I did manage to get through the NATO campaign as well as Resistance. I'm glad someone else mentioned it, and I am glad I'm not alone. The mission where they just drop you and 4 other guys on Kolgujev and say, "Go kill Shilkas," is freak'n tough!
I would also have to add Star Trek: Echoes from the Past. It's nearly impossible to beat without some kind of map and some blessings/enchantments to guide your path. I tried to beat the mine mission by myself and I just beat it two months ago (I eventually broke down and found a map online). I had been playing it since I was about 9 (I'm 22 now for reference)!!! No, seriously... Look it up if you don't believe me! It's quite possibly the hardest Sega Genesis game ever made!!!
Honorable mention: Ghosts and Goblins