For beginners, in both CK2+ and vanilla, I've always been a fan of Irish count or Poland in either 1066 start.
I think it's Dublin, your father(?) has another county, so you have an ally, a good province, and at some point you'll inherit a second province. Going from count to King of Ireland will teach you about warfare on a small scale, de jure duchies / kingdoms, alliances and inheritance, and then you'll be in a good position to expand elsewhere - Wales, Britanny, North Iberia?
Poland I like because you start young and unmarried, with almost no few immediate threats, several potential allies and several options for expansion, but the game gets progressively harder as you go on, culminating with the Mongol horde on your doorstep. There's pagans all around, but you need to beat the HRE, Denmark and the Rurikoviches (Novgorod, Kieve, etc. are all of one dynasty, the Rurikoviches) to them, and not let the pagans themselves unite (I once fought a Wendish Lithuania, which also held a massive chunk of the Baltic coast. No fun.)