Hmmmm. L was the one to outsmart and foil Light's plans most of the time, but he also overlooked something important which gave Light the advantage. Maybe Light? Idk it's hard to say for certain who iq mogs the other. I'll just say Yagami Light.
Definitely L, he outsmarted Light 1000x more from the start. Narrowed down to the correct city etc. Light barely won L even with owning the death note. On the other hand if you look at purely their motives I'd say Light is smarter.