_TIE fighters, very effectuve and menacing effect.
_Vader's breathing, probably the most quoted sound effect in the history of movies.
_lightsabers, the 2nd most quoted.....
_R2... dialogue with sound effects...great!
i also like few others:
_imperial shuttle, very cool
_the mecanic sounds of the droids
_the carbon freezing chamber's "pliers"...i love when the sound goes "aaawwwoooaaaaahh..."