I'm pretty certain that these two will be my top two luchadores, and very likely both within my top five. I'm just not sure which of the two will rank higher.
I just want to throw this out there now with a few comments, and I'll try to go more in depth later.
Satanico is everything advertised. He's simply the best rudo of all time, quite possibly the best bad guy in the history of wrestling. He's able to portray the joy of evil through his mannerisms and expressions in a way that I've never seen from anyone else. There's almost nothing in lucha I love more than the way he directs traffic in a trios beatdown. He's a technical master, with matwork full of struggle and weight. An extremely smart worker that could have one of the best matches of all time with little more than basic strikes, but that could also turn it up for quick-paced brawls or some of the most amazing title matches you'd ever see. While he has a few babyface (we'll call it that and not tecnico) runs and certainly played that role with the crowd behind him in some rudo vs rudo matches, I think, with him, it's more of a case of being the deepest pond than a wide one. He's as bad as bad can be, the very best of the very worst.
Casas, on the other hand, feels far more versatile to me. He has workrate sprints, heated apuestas matches, big title matches, feuds full of character. He's one of the most emotive, charismatic wrestlers of all times, both in his younger, athletic work and in his later, wizened trickster god persona. I don't think he's quite as strong a rudo as Satanico and at times his charisma could even be a negative because it can take away from the clarity of his matches when up against a straight tecnico. I'm not sure there's anyone nominated who has as many great performances that feel as different from one another (maybe Funk?). For someone who could play so naturally cocky a jerk, he could also garner incredible amounts of sympathy. Just an amazing range that carries on to today, where he can light up a trios match just by stepping in through the ropes. I do think there were periods in the early-mid 00s where he could, as a tecnico, fade into the background too much in trios matches, but that was him playing his role and not overshadowing the main focus on the match, so it's hard to hold it against him too much. It wasn't something I often saw with Satanico though.
Someone like Hijo del Santo feels "purer" to me, but I think these two have to be my top 2. I'm not sure it isn't a wide vs deep argument here, the very best at one thing (while being very good at so many other things) against someone who's almost the best at just about everything.