What is the most reactive to the least reactive?
Iodine, Bromine, and Chlorine

Chlorine, Bromine, Iodine.
Here is a good explanation why:

Re: Why is Flourine the most reactive non-metal? (http://www.madsci.org/posts/archives/jan2001/979441725.Ch.r.html)

Fluorine the most reactive element because it has the highest electronegativity. It's proton:shell number ratio is the most efficient of all elements.