I like Serbia the most of the poll choices. A semi-westernized kingdom, with strong infantry and its famous Serbian knights coupled with some Slavic units and horse archers.

However I think the Cumans would overall make a more useful contribution to the gameplay, so this time I voted for them.

Aragon is another faction that I'd have loved to see implemented, but now that we have Portugal there's less reason to put yet another faction in Iberia (would still be interesting though) so there are better choices IMO.