I'm 26 years old and been a fan of Athletic Club since I was born, I have gone to practically all the stadiums in Spain (except Getafe and Tenerife), most of the fans are quite passionate but you can have fun with them, there isn't much bad blood, of course there are matches and matches, RM and BCN tease and pull each other's chain a LOT but at the end of the day things are fine, you didn't see Barcelona rioting for not winning the CL last year or Madrid rioting for lack of titles in a couple of years.
There are several famous rivalries in Spain besides RM and BCN: Espanyol and Barcelona, Atlético and Real Madrid and of course the Basque Derbi: Real Sociedad vs Athletic Club.
I think I'm used to see rival fans sitting together and having a beer while waiting for the match, I've gone to Anoeta decked head to toe in Athletic gear and no one has bashed or insulted me for it.
That's how we roll in Euskadi, you can sit with rivals without any single problem happening.
I've also gone to the two biggest Madrid stadiums: Santiago Bernabeu and Vicente Calderón, the first one is not exactly very friendly but you can work it out there, the Atleti stadium is simply unbearable.
They are known as the worst fanbase in Spain hands down, they scream bad things at players, are racists, kick fans in the stands, throw stuff at the field...
This is what they do to rival players:
One of our captains, Gurpegui, gets called: "eres un yonki" (drug addict) or "pásame un gramo" (pass me a gram of drug) at the Vicente Calderón stadium ¬¬
they also sing: "vascos vascos, hemos venido a fusilaros" (basques, we've come to execute (kill) you")
"Gurpegui muerete" "Die Gurpegui die"
They have black/latinamerican players in their club but still chant this kind of shit to rival players:
"Mourinho son of a bitch" "Mourinho die" "That Portuguese is suck a motherfucker" "Marcelo you are a Monkey" "Die Pepe Die"
The Atletic gang throwing Flares to Sevilla fans.
You can find tons and tons of related issues about this fanatism, the LFP does nothing, the Spanish Football Federation does nothing, UEFA does nothing, the referees look the other way...
It's really tiresome and annoying that this situation is still allowed to happen in our stadiums, I hope something can be done but I don't count on it ¬¬