Warning re "Merlí" and "Merlí: Sapere Aude".
Both are about half in Catalan, half in Spanish.
Only mentioning because OP specifically requested Spanish language series.
I just watched "If Only" this weekend and didn't enjoy it much.
I did enjoy "The Secrets of the Greco Family" and "Sagrada Familia". I guess I like those "twisted family" stories.
Kudos to whoever mentioned "El Ministerio del Tiempo". I really enjoyed that series a few years ago, it was a lot of fun to watch. I think there was an additional season that never made it onto Netflix (dammit!)
"Unauthorized Living", "Dark Desire", and "The Mess You Leave Behind" are all fairly lowbrow, trashy soaps...but watchable, if you're into that kind of thing. I found "Unauthorized Living" borderline offensive in its portrayal of Mexicans. It's a Spanish series, but a big part of the plot involves relatives visiting from Mexico. They're all drug dealers (surprise!), they all speak in VERY exaggerated Mexican accents (all played by Spanish actors, not Mexicans), and they all behave crudely and boorishly, which is exactly the opposite of how real Mexicans from Mexico actually behave.
Anyway, my recommendations:
"La Marginal", a prison drama from Argentina. It's gritty to the point of being visually unappealing, but it's a good, compelling crime drama. I think there were 4 or 5 seasons. There was also a Mexican version, which I believe only lasted 1 season.
Also from Argentina, "Casi Feliz" ("Almost Happy"). Starring comedian Sebastian Wainraich as a radio DJ, separated from his wife but still in love with her. I found it really well done, funny and downright charming. The eps are only 1/2 hour each, so it breezes by.
From Mexico, "Fuerza Negra" ("Dark Forces"). Starring Tenoch Huerta ("Namor" in "Black Panther: Forever Wakanda"). It's one of those "silent, mysterious loner gunman" things. I'd seen Tenoch in some other things, but this was the first time I saw that he really had star potential. I know some people are now criticizing his looks and his physique, but I thought he was sexy AF in this role.
"Welcome to Eden", from Spain. This type of story has been done many times before: basically, a bunch of people find themselves in a place that turns out to not be all that it seems, and when they want to leave, they find they can't. Even though it's a bit cliched, I thought it was well done.
A couple of Spanish period pieces: "La Catedral del Mar" (The Cathedral of the Sea), and its sequel "Los Herederos de la Tierra" (Heirs to the Land). Both based on historical novels by Ildefonso Falcones, set in 14th century Barcelona. Both are great.
If you're just looking for beefcake, there's "High Heat" (from Mexico, about firefighters. They show LOTS of body, each one's hotter than the next, and there's a gay storyline), or "Toy Boy" (from Spain, about male strippers. They show surprisingly little, though, for being strippers. And one of them is gay.) Both are kinda trash, but not unwatchable.
I wanted to recommend a Spanish series called "Patria", but I realized it's on HBO, not Netflix. I loved it, though. It tells the story of the Basque conflict through the eyes of two families who find themselves on opposite sides. Elena Irureta gives an astonishing performance as one of the two matriarchs.