In ANH, the Rebels themselves are the underdogs. They represent the generic David (Rebels) vs. Goliath (Empire).
In ESB, the character of Cloud City sort of represents the "normal" guy who gets inadvertently dragged into a larger conflict.
In ROTJ, the Ewoks represent the quintessential battle of man vs. machine. "au Naturale" vs. Technology. That sort of thing.
In TPM, the Gungans represent the oft lambasted Star Trek nerd. Jar Jar is that dork everyone remembers from school who just never understands how geeky he really is. And Revenge of the Nerds style, it is the dorks who come to save the day.
But what about AOTC and ROTS?