Definitely Nathan. That's not to say Lucas never had his moments because he obviously did but like others have said, he was never very original and he recycled a lot with each girl he was with. That, to me, cuts down majorly in the romantic department. For a writer, I felt that Lucas failed miserably in an area where it should be like second nature for him - not to mention as a boyfriend he failed completely, hence the reasons I was never a fan of any of the Lucas relationships (even with Peyton who is a favorite of mine - I was a Jeyton fan).
Nathan was always sweet with Haley. He was always making these gestures (big and small) and saying these speeches to prove his love for Haley. Another reason is because it was only ever Haley. Ever since they got together, Nathan has known she was the one for him - and that is incredibly romantic. He wasn't like Lucas who fell in and out of love, jumping from one girl to the next so quickly. I'll be the one of the first to admit Nathan was sometimes cheesy but it was the good kind of cheesy for me. I also felt that most of his cheesiness was during the high school years and really, what relationship isn't a little cheesy in high school no matter how serious or casual it may be? LOL!