Unfortunately for the visitor, nearly all of Venice's museums and galleries are worth the time. Just be glad Napoleon's agents nicked everything that wasn't nailed down, or your must-sees would be much, much worse.
If you only have time for a few, you might want to concentrate on the uniquely Venetian ones: the Accademia, that great repository of Venetian masterpieces; the Museo Correr, Venice's attic; St Mark's Museum, to see the original bronze horses up close; the Naval History Museum, dedicated to the seafaring traditions of a great seafaring city; the Fondazione Querini-Stampalia, especially for its charming collection of Venetian genre paintings by Longhi and Bella; the delightful Peggy Guggenheim Collection for modern art; the Glass Museum on Murano, for insights into one of Venice's greatest industries; and the Ca' Rezzonico, dedicated to the Serene Republic's long 18th-century farewell party.
Image by Entoaggie09 Creative Commons License