Of the bigger factions England, Hungary and the Byzantines (Early) are the most enjoyable IMO, but the really interesting factions are the minor ones. Sweden, Norway, Serbia, Bulgaria, Armenia, Bohemia and Burgundy are my favourites, much thanks to their size, nice units and the difficulties it means to start with just one teched up province (and in many cases; just one province in total).

PS. Noticed that I mentioned like half of all the factions in this post