What's an underappreciated work of classic art?

Interesting! Can I ask, what are some of the fine art best-selling jigsaws?
And is there any demand for Velasquez, Juan de Pareja and Pope Innocent X, which are considered as 2 of the greatest portraits ever painted?

The only works by Velazquez that have really ever got any traction are The Spinners and Las Meninas. With notable exceptions like Mona Lisa, portraits don’t generally do well, nor paintings with muted colors. The big pre-19th Century sellers are anything found in the Vatican (Botticelli, Raphael, Michelangelo), several Bruegels, Bosch’s Garden of Earthly Delights and (kind of out of left field of late) van Os still lifes.

The Impressionists and Art Nouveau dominate the Fine Art niche of the jigsaw puzzle market; van Gogh and Mucha, especially. Klimt and Kandinsky are probably the best sellers from the 20th Century, but mostly because more popular artwork from the likes of Picasso, Dali, Kahlo, and Rivera are still difficult to license.

I know three fairly well, and have heard of two others. A couple I have never even heard of.

Ecce Homo, the botched fresco on a Spanish church from a few years back, is justly derided for its awfulness. It’s also a major contender for “most famous painting of the 21st Century so far.” The original painting that needed the touch-up apparently only dates back to 1930, not exactly the medieval icon masterpiece it was sort-of reputed to be.