The Picasso Museum is bona fide “exceptional” because of the early work they have by Picasso. There are roomfuls of small canvases and works on paper, which are on permanent display, from his early teens and adolescence; lots of small paintings and homework and studies from art school, which he kept throughout his life and eventually - upon his death - were left to his family who donated them as a single collection. So we have his student work as a young man when he was striving as hard as he could for realism. Working mainly in portraiture and on nudes of the human form, you can see him progress in a matter of years, as you stroll around the rooms, into the young avant-garde with which we’re so familiar today.
Side note... back in New York City I’ve got a good friend who likes to say Picasso was every bit as talented and artistically “good" as the Renaissance Masters when he was a young man… in “defense” of his later style, I’m told that in his youth Picasso learned to paint and draw with equal skill and mastery to the renaissance and baroque masters. But that's a myth. When standing in front of the actual youthful works you are impressed by his skill... he was really (really - no bullshit) talented. I was impressed at what he was doing when he was 14 and would say it was well beyond his years... but the work doesn't even come close to approaching the level of mastery that the teenaged Old Masters possessed. Which is certainly no dig against Picasso. There is an almost ridiculous level of technical proficiency, coupled with enormous sensitivity to psychology and form, which is the hallmark of a 14 year old Dürer or teenaged Michelangelo... we have, in the museums of the world, works by young Rembrandt and young Van Dyck... we know what youthful genius looks like in the sketches and paintings of teenaged Rubens and Raphael; and with all due respect to Pablo Picasso, this is not it.
Among the highlights of the collection are a series of 50-ish paintings that Picasso based on the Velasquez masterpiece Las Maninas. If you don't know that painting by Velasquez I highly suggest you look at it before your visit. I’ve posted it here, for your own comparison... Having seen Velasquez’s original while visiting The Prado in Madrid made all the difference in the world to our visit to Barcelona, because my wife was able to recognize the Picasso series for what it was: such a total abstraction of the original that it’s borderline just silly.
I think the highlight of Barcelona for me was my wife's reaction to Picasso's interpretations of that painting. She just laughed and laughed. Good times...