IMHO it's hard to argue that any philosopher is more influential than Plato, especially when you consider the clear neo-Platonic influences on Christian theology, and the subsequent influence that that tradition has had on Western civilization.
Elaboration: keep in mind the whole point of creating a city in words was to get a look at the soul, and Socrates hints at the fact the city wouldn't be practical in real life.
Not sure what you're referring to there, but the feudal ideas of class determined by birth and lack of social mobility were adopted from the Republic (though the ruling classes certainly took some liberties in their interpretation; some analyses of the Republic suggest it wasn't meant to be quite so rigid).
