Solo, St Mark's Museum & the Bronze Horses works.
Up a steep stair inside the basilica, this small museum holds the original gilded bronze Horses of Saint Mark (Roman loot from Constantinople) and opens onto the external loggia overlooking the Piazza — the single best close-up of the square and the mosaics. The catch: it's a paid add-on to the free basilica, the stairs are steep, and the replicas outside fool most people into skipping the originals.