This is a very, very interesting match up. I've made my choices a few times, and its always been Lelouch.
If its a physical fight:
Light is obviously stronger physically, he played Tennis and is fit. Lelouch is just skin and bones basically. But, Lelouch has the Geass, which is in his eye, therefore, Light wouldn't have a chance unless he wears a mask. But then how will he know to wear a mask?
If it's a long mental fight with unlimited resources:
Light wins no doubt. He could easily find out Lelouch's real name and write his name down. OH WAIT, Lelouch wears a mask, and unless Light finds a picture along with the name, hes out of luck. If Light messes up just a little bit, Lelouch could outsmart him and use his Geass to close the battle.
If its a right-here-right-now battle where only one survives and the two have never met/heard of each other before:
Lelouch wins. The only way Light wins is if he uses the Shingami's eyes, BUT, that requires Lelouch's mask to be off. Light wins if he can kill him physically, or knock his mask off and make the deal with Ryuk. If not, Lelouch wins easily. He just uses his Geass to have Light kill himself with the pen.