Here are some possibilities, depending on which dialect you're interested in:
* Quenya: *nílassë* (meaning "joy" or "light")
* Sindarin: *nîn* (meaning "joy" or "happiness")
* Telerin: *nílas* (meaning "light" or "brightness")
It's important to note that these are interpretations and translations, and not official, direct translations of "smile."
You can also create your own word by combining elements of Elvish, such as combining the root words for "face" and "joy".
Ultimately, the "best" way to spell "smile" in Elvish depends on the specific context and the intended meaning you want to convey.