I say Diablo II, it's easy but it's so long it gets boring, and when it gets boring then it gets kinda tough.
Also WC3, because there is no strategy in it. All the computer does is start-> build barracks->attack
I mean ftw, for me, anything which is against me, or i can't conquer then the game sucks/it's tough.
but the wc3 campaign is OKAY.
WC3 is insanely easy, WCII is way harder..
How is Diablo II hard? I beat it on all modes and barely ever died, the game was so easy!
The hardest games for me....maybe:
Metroid Prime 2
Darked Skye (SKITTLES, not hard, but I had low health and no healing items)
Metal Slug 4 (was pretty hard...)
Final Fantasy IV (If I remember correctly, I had no healing items, low mana and two dead party members at the fire boss)
Super Turican (Man, this game is amazing!)
Time Splitters 2 (story mode on the harder modes and the arcade elite)
Civilizations IV (Not hard...but it's sooo long)
Trauma Center: Second Opinion (It got really hard after a while)
Donkey Kong Country
I never really found too many games to be hard...but those might be them.