How would you write Light Yagami's name in the Death Note?
Remember that the Death Note is very specific about using the original spelling for a name.


  So which of "Tsuki" or "Raito" can kill him?


The only one that matters, 月 or "tsuki". Early in the manga he makes a point about the pronunciation for this very reason.


  Would "Light" in English work?


no. The original spelling is required.

The following will work:


夜神月


The following will not work:


夜神
月
ヤガミライト
Yagami Raito
Yagami Light
Light