In my opinion, Natsu smiles better :) But, I still like Luffy and I love the anime One piece, no hate on Luffy!
Who smiles better: Natsu or Luffy?
In this picture, Natsu. Not saying that Luffy smile is bad, I mean Luffy's smile doesn't look like a smile that is for friendship in this picture, it kinda looks like the smile of food or something. Natsu smile is good because his smile looks like he accomplished something really big and did something for friendship.