which is more reactive potassium or bromine?

Answer 1

Potassium. If you expose its pure form to even the air, it'll blow up in your face. It is very reactive with water, even the miniscule amounts in the air.

Answer 2

This depends entirely on what they are reacting with, and I do not think there is any way of setting up a competition that isn't apples and oranges.

Potassium reacts violently with water; bromine doesn't. However, bromine reacts with ethylene (ethene, as some people call it), while potassium doesn't.