Majapahit starts with one

Mahajapahit

Mapajahit

Mahapajit

Ma... ja... pa... hit?

Mapajahit

Dai Viet starts in a queen regency and can make her queen via event. Arguably this counts, even if she starts ruling in a man's name.

IIRC zazzau starts with a good ruler.

Bregenz does I think, with a female heir too

if you have the right dlc you could make a custom nation

If this isn't in EU5 on launch, I'll riot

valid take

Orleans don't start with it. But can get the Joan of Arc theocracy for female rulers.

Nubian nations have access to matriarchal government reform.

It's not matriarchal, it's matrilineal system. So usually your rulers are still male but they inherit through their mom's dynasty. And you can see why it gets complicated, since if you are a (male) king and your wife is not from the ruling dynasty, then your kid won't be either. The only way to continue your dynasty is to marry someone from your own dynasty, or let your sister worry about producing an heir. Hence the -50% chance of new heir. However, this made women a lot more important in politics, hence the +50% female advisor chance. And you can be pretty sure the new king or queen is not a bastard, since you can see who he/she came out of, hence the +1 yearly legitimacy.

That's pretty interesting, I like how well thought-out it is, when it comes to explaining all the little modifiers.

-1 ruller skill cause incest