Modern Warfare. Set the bar for story telling, graphics, set pieces, and multiplayer back at the end of 2007. It seems that the newer games could not maintain that momentum, and visually they haven't aged well because they've used the same engine for the last 4 years. In MW2, I am constantly blown away by how amazing the character models look, but other times the textures are incredibly bland and lacking of detail, and the animations are jerky. I run it maxed out at 1080p on my PC, around 70FPS, so it's not the medium, it's just the engine.
On top of this, the story made the most sense in Modern Warfare- it seemed plausible, realistic. MW2 was all over the map (literally!) with Captain Price nuking the US(?!?). The original Modern Warfare just had everything going for it.