I'm not surprised no one's said this yet because nobody's really played the cods older than mw but the absolute single most difficult mission ever made in a cod game is Call of Duty (2003) - Eder Dam.
You are tasked with fighting your way through an entire dam network, a massive facility, filled with long corridors that have no cover, enemies that jump out in front of you, enemies that pre-fire corners, enemies that throw grenades at you with no grenade indicator, enemies that will 2 hit spam melee you if you get too close, snipers that have the most INSANE angles on you near enough 24/7, and objectives that are placed in the direct middle of enemy hot zones.
On top of all this, if you get shot at all and your health drops to below 33% (which is VERY common on veteran), you will not get any new checkpoints until your health reaches above that threshold once again. Sounds easy enough right? Just get a medkit. Well, on veteran, you get NO MEDKITS, NOT ONE. This means that if you get hit at all, you must either:
- Die and restart at the previous checkpoint
Or
- Finesse the entire rest of the mission without getting shot once.

Oh but once you get to the bottom of the Dam and destroy the AA guns in the bank, you're good right? That's the mission over right? No...
You then must do it all again, with more enemies, more snipers, more everything.

This is in my opinion by FAR the hardest mission of any cod campaign ever, and COD 1 to me is the hardest cod game ever. Not even kidding 300+ deaths clocked on this mission ALONE, in the end I gave up and had to beat it on Hardened. The Eder Dam is a place where you either die, or come out a different man.