What is a good anime series for someone who likes code geass and nabari no ou .

I'll recommend: Death Note because if you like Code Geass you'll love that. I'll also recommend FMA

Rurouni Kenshin

Fullmetal Alchemist, Loveless (If you don't mind slight yaoi), 07 Ghost, maybe Ghost Hunt, and that's all I can think of right now...
Those animes have the genre military, supernatural, action, or all of them.

Those are 2 of my fav's as well so u may also like:
Angel Beats
Blue Exorcist (Ao no Exorcist)
07 Ghost
Another
Durarara!!
Darker Than Black
Trinity Blood
Fullmetal Alchemist Brotherhood
Death Note

I think of Death Note when anime similar to Code Geass is mentioned (though I personally prefer Code Geass). I also recommend Gurren Lagann and Evangelion if you enjoyed the mech aspect of it.

I think a person who enjoyed Nabari no Ou would also like Pandora Hearts, Loveless, and maybe Soul Eater...?