The elements that would react most easily with each other would be "an element with a high electronegativity and an element with a single valence electron."
This is because an element with a single valence electron (typically from the alkali metals group, like sodium or potassium) is eager to lose that electron and achieve a stable electronic configuration, while an element with high electronegativity (such as the halogens) is very eager to gain electrons. This creates a strong driving force for the two to react chemically, forming an ionic bond.