So far I think the satanism mod may be my favorite.
It's not quite complete, and it doesn't naturally occur in game, so any satanists have to be made in ruler designer, (I would've made it a Catholic or Orthodox heresy personally, but it's creator made it a stand alone faith) but the features it does add have easily made up for the weaknesses inherent in being despised by all other faiths and are pretty fun in a cheesy over the top HAIL SATAN kind of way, a bit like the Child of Satan event but with better soldiers and less familial murder.
Sacrificing a child and 500 piety for 666 very powerful, very expensive to maintain soldiers has given me the ability to fight armies several times my size and wage near unending war (which I need to, without raiding my savings are depleted in very little time). All that's been holding me back is finding more enemies I can wage Holy Wars on.
Starting as the Emir of Jerusalem in service to the Abbassids, I've now carved a vast empire out of the Byzantines, Abbassids, Georgia and the tribal lands to the north of Georgia. I'm the biggest single nation around, the most beloved ruler in the world, my descendants are already beyond counting and my armies number in the tens of thousands.
I'm over 100 (super high health stat from character designer) and have outlived more than half of my children and several of my grandchildren, castrated a Byzantine Emperor, stole the wives of an Abbassid Caliph, outlasted half a dozen of the Caliphs, instituted Elective Monarchy and Absolute-Cognatic succession in two of my kingdoms, though the third can't be changed yet and the Empire is not yet Cognatic.
All it cost me was 3000 piety and the lives of one of my sickly children, 2 bastards my unfaithful concubines bore for my vassals and three captured children of the Abbassid Empire. My legion of hellspawn has never lost a fight, though attrition of all things has felled many of them and they can only be in so many places at once.
Even the Giant, Lunatic Werewolf Viking was less fun than this, and he fought Cthulhu.
I'm actually somewhat curious as to what the maximum potential lifespan someone can have is. I know high health lets you live longer, but I don't know what the random equation used is, or at what age health simply can't overcome the penalties to your death tests.
Also, I wish there was a way to mass ransom people. Constant war and raiding has left me with a prison filled with 70+ inmates (the highest point was 97, but they've been dying of age and diseases) and I can't really be bothered to process them at all.