* τέχνης (téchnis): This refers to "art" or "craft," and can be used broadly to describe an artist, especially one skilled in a particular craft. You might say someone is a "τέχνης" (a person of art/craft).
* καλλιτέχνης (kallitexnis): This is probably the closest single word and is generally understood to mean "artist," especially in the fine arts context (painting, sculpture, etc.).
* δημιουργός (dēmiourgós): This means "creator" and is a good choice if you want to emphasize the creative process. It's suitable for a wide range of artists, but it might be too broad depending on the context.
The best word to use will depend on the context. For most situations, καλλιτέχνης (kallitexnis) is a safe and accurate choice.