If you could only go to one museum in Florence, would you go to the Uffizi or the Academia?
Uffizi, hands down.
If you like Italian art, go to the Uffizi.
If you don't like Italian art, go to the Uffizi.
Academia is really a fast museum to see, if you don't want to miss the Statue of David it would not take much time to see, and you can buy online tickets with a specific time of entrance.
I like Kent's answer!
Yep, Uffizi it is.