This is an interesting question, though I'd have to say more than likely L was the more intelligent one. Each of the two had their own weakness that got in the way of their intellect though. L, while extremely intelligent, had an anti-social--or just a lack of how to be sociable. He seemed to only understand hard facts. This, in a way, led to his downfall. Light, all his own, was too arrogant. His intelligence and capability to kill L by taking advantage of that fact made him arrogant and careless. This too, led to his downfall--but he was more of his own cause, whereas L was not as much at fault for his own downfall. To the best of his abilities, L often corrected his faults to the best of his abilities, where as Light [Kira] overlooked them, and thrived upon them.
An equally interesting question is who was truly in the right or wrong? Light or L?