Very few, and it's been a while. I really have to love a game to play through it on the hardest difficulty. So...
Doom/Doom II - Ultraviolence (not Nightmare)
Fear Effect 1 and 2
Red Alert
Civilisation II - conquered the world on Deity.
Disruptor
Of those I'd say Civ II was the hardest, due to the AI being insanely agressive and blatantly cheating.
Funny thing is, once I've beaten a game on it's hardest settings I never go back to the easier/normal settings because it becomes too easy.
Doom/Doom II - Ultraviolence (not Nightmare)
Fear Effect 1 and 2
Red Alert
Civilisation II - conquered the world on Deity.
Disruptor
Of those I'd say Civ II was the hardest, due to the AI being insanely agressive and blatantly cheating.
Funny thing is, once I've beaten a game on it's hardest settings I never go back to the easier/normal settings because it becomes too easy.