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 :)