The only time I have seen Sasuke smile was to Karin. And it doesn't have anything to do with the pairing here but it was the only time he smiled, his eyebrows weren't tensed, so to speak, it was that smile he had when his parents were alive...
The rest of the smiles might be considered 'good smiles' however, the most genuine and truthfully I find the one he gave Karin.
his face was genuinely light and absolutely no tension.