Mona Lisa, Educa "Miniature" series, 1000pcs. This is hands down one of the hardest because every piece fits perfectly with every other piece, and since it's tiny, it's hard to discern the
Amazon.com: Educa Boras Guernica P. Picasso Miniature Puzzle (1000 Piece) : Varios: Toys & Games
Amazon.com: Educa 14835 Miniature Cans Jigsaw Puzzle, One Color : Toys & Games