You should never have Flora or Shadows on.
But always keep Fog Shadows as it impacts Night Vision scopes. Otherwise, just keep everything the same. If flying, you should probably have render distance higher than 800 or so, but if purely infantry, less than 500 is fine.
Motion Blur is a preference. I like it as motion is easier to detect for the human eye, making enemies even easier to spot. Ambient Occlusion should be turned off unless you can somehow still get it to run with a consistent 60 FPS.
Graphics (the option, not the overall graphics) should be High or Ultra as some items don't show certain things unless they are at that level (see the Claymore). Textures can be Low if you don't care for the eyecandy (even at High its not that great up close). Lighting should usually be Low unless you can deal with dynamic lighting (which is extremely useful for spotting tracers of weapons).