So far I've played 1-3, doing 3 on Veteran right now. I have MW1 and 2, and WAW after these.
Call of Duty Classic, no regenerating health, and no health pickups, so you are stuck with the one bar the entire level, and you die in 1-2 shots. Plus checkpoints won't happen unless you have mostly full health.
Call of Duty 3, infinite spawning, and fellow solider AI is much worse then it was in Call of Duty 2, I don't know how may times I've seen fellow allies shoot walls, and the cover they are behind.. Plus enemies can shoot through solid objects and cover a lot of the time, so many deaths feel really unfair. Also checkpoints are really far apart.
And the easiest was Call of Duty 2. It was tough, but not so tough that it still wasn't fun, CODC and COD 3 were/are frustratingly tough, so they are no longer fun.