Where: Museo Nacional Centro de Arte Reina Sofía, Madrid (Calle de Santa Isabel, 52; +34 917 74 10 00).
Four of the 15 most expensive paintings ever sold are by Picasso, yet the work widely regarded as his greatest masterpiece would almost certainly smash all records if put up for auction.
Guernica's size -- 3.5 meters by 7.8 meters -- makes it unusual, but it's the political commentary about Nazi bombing during the Spanish Civil War and the layers of symbolism piled within that make it arguably the most famous artwork of the 20th century.
What: The Parthenon sculptures.
Where: The Acropolis Museum, Athens (15 Dionysiou Areopagitou Street, +30 21 0900 0900)
There's no greater symbol of Ancient Greek classicism than the Parthenon, and most of the treasures from Athens's hilltop temple are inside the Acropolis Museum.
Dr Tom Flynn, author of "The Universal Museum," says the Parthenon sculptures are the pinnacle of Greek high classical sculpture.
"Their location in the Parthenon Gallery of the New Acropolis Museums allows us to appreciate them in the context of the Parthenon itself. You can faithfully recreate their original disposition on the temple, which is visible from the gallery," he says.
What: Mantegna's "Dead Christ."
Where: Pinacoteca di Brera, Milan (Via Brera 28; 00 39 02 722 631).
"Leonardo da Vinci's 'Last Supper' is Milan's outstanding artwork, but it's on a convent wall, not in a museum," says Donald Strachan, co-author of Frommer's Italy.
His favored alternative is Mantegna's "Dead Christ," which was regarded as genuinely shocking when painted in the 1480s.
"It is brutal in its simplicity, unsentimental in its treatment of Jesus's pallid corpse on a slab and a masterclass in realistic foreshortening, which was a relatively recent innovation in art," Strachan says.