I've not looked across all 6 pages, 'cause I detect some arsey 'only noobs use pads!' comments, so I'll just throw my selection into the ring firing range: Black. Not because it was tougher than COD4 on Vet (it wasn't) but because it took bloody ages to clear any level... It became a battle of attrition, as you burned through thousands of rounds across an hour or two of a single map.
I think I only ever saw the ending once, because that last level was so bastard hard (and long). It was a phenomenal FPS - and it deserves a proper sequel - but I don't think I've ever found a game that simply wears down your will to live as much as Black.
Failing that, S.T.A.L.K.E.R. can be punishing, and COD4's Mile High Club achievement was a worthy end to a very challenging game.