One word: David. Michelangelo's most famous statue is sheer perfection. If you're visiting Florence, you
HAVE to see this work of art. There are copies elsewhere in the city (Piazza della Signoria & Michelangelo, to name two) but you'll be kicking yourself if you don't get to see the original.
Also of note in the Accademia are Michelangelo's "Slaves", conveniently located in the same room as "David". However, the paintings can be safely skipped.
To avoid lineups, go about an hour before closing. Since there are only two noteworthy things to see here, that should be more than enough time.
Warning: photography is banned in the Accademia but there's only one guard to enforce the rule. Form a temporary alliance with your fellow tourists to distract him/her while you snap away!