Depends on your start date. If you start 1066, you start as the Duchy of Rashka, and get to create the Kingdom of Serbia on your own, which is a huuuge amount of fun. If you play your cards right, the ERE should splinter, allowing you to prey on their remains: however, your other hand has to be holding off Croatia, and if Hungary declared war... Yeah. You might wanna gang-bang the Pechenegs a little bit. Unhistorical, but a good way to gain in power quickly.
Not so familiar with the other scenarios, sorry.