The most reactive among the halogens is

Among the halogens, reactivity decreases down the group because atomic size increases and the ability to attract an additional electron (electronegativity) falls; a stronger pull on electrons makes an element a more powerful oxidizing agent. Fluorine is at the top of Group 17 and has the highest electronegativity, making it the strongest oxidizer and thus the most reactive halogen. Pauling-scale values show fluorine near 4.0 while chlorine is substantially lower, which quantitatively supports fluorine’s greater tendency to gain electrons and react [1]. Therefore fluorine, with its very high electron-attracting power and small atomic radius, is the most reactive halogen.

Sources

1. [1] https://roymech.org/Related/Chemistry/Chem_Electronegativity.html