My favorite Special effect is when the Tantive IV gets pulled into the underbelly of the Star Destroyer in A New Hope. I love how the Imperial ship seems to sprawl out over the tiny rebel ship when in fact the Blockade Runner model was much larger than the star destroyer since it was originally supposed to be the Falcon.

Ooooh. And also when Luke loses his hand in TESB. This is still the only limb loss in all six movies that is convincing. (and yes I'm talking primarily about Anakin's little "here let me hold my whole arm out and hope it doesn't get hacked off by this old guy" AOTC mistake.)