My all time favorite is:
"Luke: I don't... I don't believe it.
Yoda: That is why you fail."
Other memorable ones:
"And well you should not. For my ally is the Force. And a
powerful ally it is. Life creates it, makes it grow. It's energy
surrounds us and binds us. Luminous beings are we... not this crude matter. You must feel the Force around you. Here, between you... me... the
tree... the rock... everywhere! Yes, even between this land and that
ship!" - Yoda
"May the Force be with you... Always." - Obi-Wan Kenobi
"Kid, I've flown from one side of this galaxy to the other. I've
seen a lot of strange stuff, but I've never seen anything to make me
believe there's one all-powerful force controlling everything. There's
no mystical energy field that controls my destiny." Han Solo
"Yippee!" - Ani