Personally I'd be putting my money on a Culture GSV or GCU. As well as being veryvery big, insanely fast/ powerful/ well armed and unimaginably clever, they have a wicked sense of humour.
An interesting quote I came across:
"Nahhh, If the Culture is the USA, the Homomdan Empire is the USSR. The Galactic Empire is the mugger on the corner of Fifth and Main and the UFP is the bum he's curb stomping." form
http://kier.3dfrontier.com/forums/ar...dex.php/t-8191