This takes a bit of thought-- my favorite upgrades:
Hemingway Suite at the Gritti Palace, Venice
Windsor Suite at the Ritz in Paris
Presidential Suite at the Hassler in Rome with my family. Oh, that glorious terrace where Eleanor Roosevelt painted, grape vines grow and my four young sons were treated as little princes. To this day, they love looking at photographs of the Spanish Steps as their terrace is on every photograph & postcard. I like this topic; makes me think and revisit great hotels, experiences and upgrades.