Answer:
Alkali metals and halogens are the most reactive, because alkali metals are group 1 elements and have one valence electron which they lose easily. And halogens, group 7 elements, have 7 valence electrons and easily gain another.
Both are more reactive, and both count as a correct answer, but I'd go with alkali metals because their reactivity increases down the group, and the elements are highly reactive with water, air and acids.