Merchant republics and theocracies are great.
Great for raiding. They tend to have either a city or temple as the main holding in their capital county, which means fort level 1-2 instead of 4-5, so it's much easier to get to the juicy stuff inside the crust. I've financed much of my assassination benefit program by looting Rome, Venezia and Genoa every once in a while.
My current game I've just completed the creation of the full Scandinavian Empire, extending beyond the de-jure realm into the baltics and Belo-Ozero, Prussia, East-Anglia and the wealthy county of Brugge. The Ynglinga dynasty has spread even further, with Granada and Portugal being ruled by an odd branch of Sunni Vikings.
I have a major problem now though. Emperor Thorgil I the great is nearing his 60th birthday and it's time to prepare for succession. He has three sons. The eldest, Thorgil, is an excellent heir to the dragon throne who has been leading armies most of his adult life. The youngest, Folki, is also a capable noble and well-suited to receive a kingdom.
The one inbetween though... Hjalmar is an imbecile, cruel, gluttonous, wrothful, shy and slothful, giving him the stats 0, 0, 0, 0, 6. I've been trying to kill him off for years, but nothing takes. Even tried to give him a temple only to discover that that trick doesn't work anymore. Maybe I should just give him a duchy now and hope his vassals deal with him quickly enough...