I'd say the Borg are at the top of the list, then the Dominion, then most likely the Klingon Empire (given the over-emphasis on service to the empire and being a warrior), after them I'd say the Federation and Romulans were about the same.
Then you have the likes of the Breen, Tholians, Tzenkethi and Gorn to think about, none of who we know that much about.
Commander Austin Harris, First Officer, Deep Space Nine (by FltCpt. Bossco)
8.01 - Darkest Before Dawn
(Chapter 8 added, 12/09/2015)