The Director's Cut is the vastly superior version - so much so that I don't think anyone even needs to see the original version any more. The removal of the narration and 'happy ending', along with the insertion of the unicorn dream sequences (hinting that Deckard himself is the final replicant) not only adds to the atmosphere of the movie but also means it makes more sense and is much more re-watchable. The emergence of the characters of the replicants - at once both psychopathic and child-like - is a central theme, best expressed at the end of the film by Rutger Hauer's character:
"I've seen things you people wouldn't believe. Attack ships on fire off the shoulder of Orion. I watched C-beams glitter in the dark near the Tannhauser gate. All those moments will be lost in time, like tears in rain. Time to die."
It is unquestionably one of the most atmospheric films ever made, and while the storyline is simple (kill the replicants one at a time) the overall effects - even without the twist for Deckard himself - draw you in to a sense of empathy with the characters, even a sense that the replicants are the most 'human' of them all. It captivates your senses, it makes you think and it leaves you asking questions.
What more could you need?