My best game began pretty conventionally as a Slavic count in the Kingdom of Great Moravia in the 867 start, I managed to gain independence and subjugate all of Poland by 900, then Bohemia and Pomerania and eventually Lithuania after a series of bloody wars. I formed the Wendish Empire, giving away Bohemia, Pomerania and Lithuania to loyal vassals, and by 1098 I reformed the Slavic faith and made it my mission to spread it as far as I could. I quickly conquered Ruthenia and Finland, granting both of them independence under Russian and Finnish rulers, then over the next few centuries did the same with Denmark, Sweden,Norway and Rus, while also conquering all of Germany, Frisia and Bavaria from the Karlings and keeping them under my rule and fighting off crusades: I eventually gave all three independence as well, although I kept supporting them in defensive wars. The game got really interesting after this when I decided to claim the Aztec Empire to spread the faith all over the world, and I ended up fighting a long war against them which I eventually won but at the cost of my current Emperor which forced me to change the succession to elective to avoid a civil war, after which I was able to install Slavic nobles in parts of Spain, Ireland and even Mauretania, all of whom managed to convert most of their land, but my armies ended up exhausted from all the fighting and I was attacked by the King of Bavaria while also facing a rebellion which took away my title as Emperor and reduced me to the King of Poland while the King of Bohemia became Emperor and I was forced to release Pomerania and Lithuania permenantly before declaring independence from the new Emperor: when he died, the title just disappeared for some reason. I spent the rest of the game trying to regain some of my lost lands, eventually reclaiming Pomerania and Bohemia by 1453, and overall the whole rise and fall with my Empire gradually becoming less powerful within the Slavic world made it the best game of CK2 I have ever played.