If the question is about performance, it's a tossup between the Spitfire and Me-109. If the question is about relative importance, it's a tossup among those two and the Hurricane. Hitler wanted to conquer and occupy Britain, but to do that he had to eliminate the Royal Air Force. Although the Luftwaffe's bombers were the planes dropping the bombs, they were sitting ducks for RAF fighters, so what the Battle of Britain amounted to at its essence was duel between the RAF's and Luftwaffe's fighters (i.e., the whole Battle was decided by fighters, not by bombers), a duel which the Luftwaffe failed to win. That is, the RAF and Britain "won" the Battle for the simple reason that Fighter Command remained a fighting force till the Luftwaffe gave up its attempt to knock out the RAF, switched to night bombing of cities, and permanently postponed its invasion (Operation Sealion). While the Spitfire and Me-109 were roughly equal to each other, the Hurricane was just a notch below, i.e. Hurricanes with competent pilots were capable of contending with Me-109s when necessary.