I would say the generic barbarian factions, Briton, Gaul and Dacia.
I think you hit the nail on the head with the problem there Mikeus - some factions in R:TW are just so GENERIC and completely lack any character at all. Not only does this make them hideously boring and repetitive to play, it makes them rubbish in combat as well.

I've actually noticed a correlation, the further a faction gets from the South-East of the map (in general) the more generic and boring it seems to get. All 7 wonders of the world are in the SE too. Compare Egypt, Seleucid Empire, Parthia to Britain, Gaul, Germania.

I don't understand how the designers thought that if Britain had developed well enough as a civilisation to have conquered Rome, Germania, Gaul and Iberia (like I have done) it would have kept the exact same set of units as when it was just in control of Britain. Surely it would have adopted units from these factions into their own line up.
Also, for Britain to have dominion over these vast lands, surely for pure logistical purposes they wouuld have had to adopt better road systems and larger cities. A permanent empire of such size would surely have otherwise folded.