Plagens makes a plenty convincing case. You can’t pick Les Dems and go wrong. But I’d still take Matisse’s 1907 Blue Nude, the painting that was so revolutionary, so avant-garde that it pushed Picasso to make Les Dems.
The first account of Picasso encountering Blue Nude comes from an American art student named Walter Pach, who included this encounter in his 1938 memoir Queer Thing Painting:
“Does that interest you,” asked Picasso. “In a way yes…” [Pach replied.] “It interests me like a blow between the eyes. I don’t understand what he is thinking,”
“Neither do I,” said Picasso. “If he wants to make a woman, let him make a woman. If he wants to make a design, let him make a design. This is between the two.”
And of course Les Dems is too. Jack Flam also wrote about Les Dems as springing from Blue Nude, pointing out that while Picasso certainly took something from African sculpture (as Picasso oft claimed), he was only able to do it after Matisse showed him how.
Flam also notes that Picasso remained obsessed with Blue Nude for many decades. He points to 1934’s Nude in a Garden, and I’d add that there are Blue Nude-referencing figures in Picasso all the way through his Women of Algiers series: In this 1955 example Picasso makes his reference to Matisse as clear as possible.
Sure, Les Dems is more famous. But Picasso needed Blue Nude to make it, and for decades thereafter.