I think Lucas is better brother,because he has always been there for Nathan.They argued in s1,but on the rest of the show Lucas has always been there for Nate.In s2 when Haley left Nathan Lucas supported him and tried to be with him,so that Nathan would't feel so much pain.And always when Nathan and Haley argued Lucas always tried to help Nathan to be with Haley again.In s5 when Nathan was drunk ass Lucas was Jamie's father and uncle.
Nathan cares about Lucas too,but to Nathan Haley is most important thing,even more important then his brother,which is logical because Lucas and Nathan didn't grow up together and they had bonded later.

^^ yeah I agree, I think Lucas was a better brother to Nathan than Nathan was to Lucas (not that Nathan was a bad brother at all).

Lucas was even willing to take the fall for Nathan's point shaving in season 4.

Lucas is the better brothet than nathan