I second Apulia. You have lots of dirty muslims to fight against against, you can strive for dominance in Italy, and later of the whole med. It's a nice compact realm with lots of opportunities.
I learned the ropes of CK as Poland, which I think is good as well. It's reasonably sized, but still managable. You also have great opportunities for early conquest at the expense of Baltic pagans. After that you can fight for supremacy with Germany and Russian princedoms.