Two you must book, and three you probably needn't
The Uffizi and the Accademia are the two that sell out, and in season they should be reserved. ⛔ Note that the queue for a ticket and the queue to get in are two different queues — having a ticket does not put you at the front of the second one.
The Bargello, San Marco and the Museo dell'Opera are rarely a problem, and between them they hold Donatello's David, Fra Angelico's cells and the original Gates of Paradise.
⭐ If the Accademia is sold out, go to the Bargello. It is the better sculpture museum, and the David there is the one Donatello cast a lifetime earlier.
Do not stack the big two
⚠️ The Uffizi and Palazzo Pitti cannot be enjoyed on the same day. Both are enormous, both are dense, and the second one becomes a corridor you walk down.
The Pitti is really several museums in one building — the Palatine Gallery, the royal apartments, costume, treasury and modern art — plus Boboli behind it. Give it its own day.
⭐ Late afternoon at the Uffizi is markedly quieter than the morning, and the light down the west corridor is better.
The Monday problem
Nearly every major museum here closes on Monday: the Uffizi, the Accademia, the Pitti, the Bargello's schedule is irregular, and several others rotate closures.
⭐ A Monday is the right day for the churches, the markets, the hills, or a day trip — and the cathedral is free and open.
The Uffizi
The Accademia
The Bargello
Palazzo Pitti
San Marco
Museo dell'Opera del Duomo
Palazzo Vecchio