Halo 2-3-ODST on Mythic Difficulty.
-Black eye Skull: requires you to melee enemies to get your shield back.
-Mythic Skull: Enemies health and shields double.
-Thunderstorm: Increases all enemies to the highest rank.
-Catch: Enemies Spam Grenades CoD style times 2.
-Tilt: Enemies resistance/weakness increases.
-Fog: Removes motion tracker.
-Blind: Removes your HUD.
-Famine: Eneimes Drop less ammo for their weapons.
-Iron: No checkpoints in single player. In CoOp one death = revert to checkpoint.
(Not a full list. Some skulls exclusive to Halo 3)
Killzone 2 on Veteran
Doom on Nightmare
Operation Flashpoint I would put it on the list, but I played user created levels way harder than the single player. So your milage may vary.
-Black eye Skull: requires you to melee enemies to get your shield back.
-Mythic Skull: Enemies health and shields double.
-Thunderstorm: Increases all enemies to the highest rank.
-Catch: Enemies Spam Grenades CoD style times 2.
-Tilt: Enemies resistance/weakness increases.
-Fog: Removes motion tracker.
-Blind: Removes your HUD.
-Famine: Eneimes Drop less ammo for their weapons.
-Iron: No checkpoints in single player. In CoOp one death = revert to checkpoint.
(Not a full list. Some skulls exclusive to Halo 3)
Killzone 2 on Veteran
Doom on Nightmare
Operation Flashpoint I would put it on the list, but I played user created levels way harder than the single player. So your milage may vary.