For me, it's got to be Operation Flashpoint: Dragon Rising.
That game is/was such a letdown when you compare it to the original Operation Flashpoint. I had been waiting for a sequal to the original OFP since its release back in 2001. When I first played Operation Flashpoint: Dragon Rising, I was shocked by what I saw. It is a messy console port with bad multiplayer, idiotic AI, zero realism, and so-so graphics. Seriously, I am angry at Codemasters for releasing that heap of crap.
They marketed it as having the realism of the original game - One bullet kills about 99% of the time, weapon recoil, sound occlusion, bullet drop, speed of sound modelling, material penetration, etc. Very little of that can be found in this game, and what is there, is modelled poorly. Recoil is almost non-existant (especially when looking through optics), I have yet to be able to shoot through a wooden door and kill someone, and I have yet to die of a single bullet to the head - it usually takes three or so.
EDIT: Edited for readability.