The Bohemians and Portuguese are standard favorites of mine, although you may find them to be a bit challenging (I have yet to take the poor Bohemians very far in my campaigns!). The Scots are fun, especially if you want to smack the English upside the head a lot. The Norwegians can be interesting, although they're sort of a one-trick pony, if you will -- far and way their best strategy it to just conquer Scandanavia very early on, starting with Sweden and working your way clockwise.

A number of people enjoy playing as the Cumans, as they have a fairly unique unit roster that includes Bashkorts and Cuman Heavy Cavalry (which are probably the best HC unit in the Early game). Quite a few players like the various Christian knight Orders in the High period, particularly the Teutonics and the Crusader States (and Maloncath currently seems to be having a great time with his Hospitaller campaign as well). And then there's the Genoese, who have the potential ability to rival Venice and the Byz in becoming a great maritime trading empire....

As you can see, there are any number of factions that are worth checking out. It really depends on your playing style, and/or if you wanted to try something different from what you usually do.