Now the best sort of smile is the one that comes naturally out of joyfulness. It is the light of your whole face, and your whole face does light up by your smile.

I personally am drawn to this kind of smile. It is attractive because it brings joy to those who see it. I tickle my girlfriend because it makes her smile and laugh in such ways.

Maybe this is irrelevant, but here is:

My generalized point of view:

I think that if a guy tells you that you have a “sexy smile”, they just want to see your reaction, because many women like to “feel” sexy. This way they can know how to approach the attraction that likely initiated their action.
If you act shy or embarassed by the compliment, they may think you are being flattered and continue to try and win you over this way. If you act strong and confident and simply thank them kindly, and then ignore them: they will give up if they are weak or dont really want you that much; or If they are so inspired they may try harder to win your affections. If you start to flirt with them at such a statement, they will think you are easy and probably not respect you very much. I would suggest simply starting an intelligent conversation with them to see what kind of person they are, and don’t be afraid to be yourself so at least you’ll know they like you for you.