All factions you've played fulfill the requirements I mentioned:
England starts in 1066 post-norman conquest with one of the most durable and strong dinasties of the game, IMHO. And after all the saxon noble start revolting, you can land most if not all of your family.
Hispania, as it doesn't exist in 1066, has to be created. All formed empires are strong and usually start post-early game, so you will have a lot of landed relatives.
HRE and ERE, despite appearances, are pretty easy and manageable, if you choose the right nobles to play with.
Like I said, primo works every time. Always. Elective only works in certain scenarios. Try playing France, Scotland, Sweden, Norway, Hungary, Poland, any minor count, any weaker duke (I recommend Duke of Anjou starting in 1066, it's a lot of fun)... those are some choices that simply don't work with elective.
EDIT:
I should point out that King of Ireland - or any created title above duke, for that matter - makes for an easy time using elective, since the prestige boost given, and the power it usually takes to do such things, makes it a breaze to manage elective.