There's no single "Greek numeral" for 666. The ancient Greeks used a system where letters represented numbers. To write 666, you'd need to use a combination of letters:
χςς (chi, stigma, stigma)
* χ (chi) = 600
* ς (stigma) = 60
* ς (stigma) = 60
Therefore, χςς = 600 + 60 + 60 = 666