* Κελλί (Kelli): This is probably the closest and most straightforward transliteration. It uses Κ (K), ε (e), λ (l), λ (l), and ι (i). This would be pronounced more like "Kel-lee."
* Κέλλη (Kelle): This uses the eta (η) which represents a long "e" sound, potentially capturing a slightly different pronunciation depending on how the name is spoken.
Ultimately, the "best" spelling depends on the intended pronunciation and the level of accuracy you're aiming for. Neither is definitively "correct."