But I'll give you an easy incorrect one: his 'tripartite theory of the soul'.
See what really gets me about people still recommending Plato is that they'll encourage others to go read some (now) useless theory like that, when they could instead be reading some modern cognitive science which covers the same ground, but with thousands of years of progress.
It's to my mind similar to telling someone to study an old doctrine about the world being composed of earth, wind, and fire instead of learning modern Chemistry.