One of:

• eyes to see a wider spectrum of light (infrared to gamma-ray)
• ears to increase sensitivity or reduce it close to 0 a will
• nose to smell all dangerous stuff that is odourless (carbon monoxide f.e.)
• muscles not to lose training after a few days (I am not sure that wouldn’t fuck up methabolism)
• have an organ to catch radiowaves (while being capable to switch it off)
• brown fat cells to never feel cold (with added super bonus that they also kill cancer by starving it)
• liver to remove toxins without being easily damaged

The choice is difficult :)

Many good ones were already mentioned

But from memory:

• The snatch
• The big Lebowsky
• Clerks
• American Psycho
• 2001 a space odissey
• Blade Runner (possibly the directors cut)
• Apocalypse now (possibly the redux version)
• Full metal jacket
• The godfather (first 2 movies, the rest is not as good)
• Fight club
• Alien
• The Truman show
• In the mouth of madness
• They live
• The terminator (first 2)
• Animal house
• the dollar trilogy from Sergio Leone
• Once upon a time in America
• pulp fiction
• reservoir dogs