Lelouch, he is superior in tactical warfare, where chess plays a bigger role.
Not a chance, Light is much more meticulous than Lelouch. L was light years ahead of Lelouch in deductive reasoning, something vital to detective work.
If L was an extreme diff, Lelouch would be mid diff in the same situation given Light holds the advantage with the supernatural powers.
Depends, but Light more times than not, he is smarter the way he goes about things in general. Lelouch leaves a bit too much room for random variables to take place, he relies a tad too much on luck and is forced to abuse his OP ability. Light makes sure luck plays as small a role as possible, he sees to it that he controls as many variables as possible, where random events are minimized.
However if both were commanding armies as generals in a battle, I'd give it to Lelouch, he is much more experienced there and would be able to read the flow of the battle at a higher level, Light would be overthinking himself into a corner in such an upfront large scale showdown.
But those scenarios are few and far between, overall Light would edge out in most scenarios, due to the difference in attention to detail.