With which group of elements is magnesium most reactive?

Magnesium is most reactive with in Group 6
Reactivity is determined by the number of an element has. When you look at Magnesium on , you see that it is in the 2nd column, and therefore has 2 .
Elements attempt to reach Noble Gas configuration (8 valence electrons) when they bond, so Magnesium is the most reactive with elements that have 6 valence electrons. These are the elements in group 6: Oxygen, Sulfure, Selenium, etc.
Credit to Wikipedia for the image