* Λεονάρδος (Leonardos): This is probably the most common and readily understood transliteration. It's a fairly direct phonetic approximation.
* Λεονάρντο (Leonarto): This version is slightly closer to the Italianate pronunciation sometimes used for the name.
The best choice depends on the context. For general use, Λεονάρδος (Leonardos) is the safest and most widely accepted option.