Lews Therin was one of the greatest channelers of his time. And he lived in a time where everyone who could channel got spectacular training in concepts and skills the people of the Third Age can hardly dream of. In Rand's case, whatever he couldn't work out for himself he had to learn from a unwilling teacher or "remember" from his past lives. So yeah, I don't think it's very surprising that Rand isn't as good as Lews Therin. Rand is also pretty much insane, and Lews (as far as we know) didn't jump off the deep end until the Taint. That's gonna affect the rankings a bit, I think.