I am biased, i will always say the first one (Episode 4) as i was into it on a big way at little school, i used to trade star wars bubblegum cards in the playground and still have that same original set. However i was astounded by Revenge of the Sith, it was awesome; plus i've been waiting many many years to see the origins of Darth Vader
I've always liked Episode V: the Empire Strikes Back. ROTJ would be the next on my list because of some of the new characters that were introduced but because of the "Jim Henson" looking stuff it started to get cheesy.
Now ive seen all of them, id say Episode IV. my fave for a long time was Return of the Jedi, because it was the first film my dad took me to see at the cinema... Aww, the memories..