Potassium is more reactive.
Alkali metals are the most reactive metals, Francium is the most reactive metal. This is as it has many layers, the last layer having only one electron, far away from the positive pull of the proton. This metal is very rare and hard to see so the next Alkali metal closest to it is Lithium. Read more at wiki.answers.com
Lithium is the most reactive.not potassium