Over its long history, the show and its spin-off media (particularly the audios by Big Finish in case of my personal preferences) has produced a lot of great dialogue. Here are just a few of my favourites from the tenure of William Hartnell, the original Doctor Who.
"Have you ever wondered what it would be like to be wanderers in the fourth dimension? Have you?"
-the Doctor, An Unearthly Child (1963)
"You wanted advice you said. I never give it. Never. But I might just say this to you. Always search for truth. My truth is in the stars and yours is here."
-the Doctor, The Rescue (The Daleks) (1963)
"One day, I shall come back. Yes, I shall come back. Until then there must be no regrets, no tears, no anxieties. Just go forward in all your beliefs, and prove to me that I am not mistaken in mine."
-the Doctor, Flashpoint (The Dalek Invasion of Earth) (1964)
"That is the dematerialising control, and that, over yonder, is the horizontal hold. Up there is the scanner; those are the doors; that is a chair with a panda on it. Sheer poetry, dear boy! Now please stop bothering me."
-the Doctor, The Watcher (The Time Meddler) (1965)
"So you're my replacements — a dandy and a clown!"
-the Doctor returns to aid his future selves, The Three Doctors (1972)
"Ah, yes! Thank you. It's good. Keep warm."
-the Doctor's last words, The Tenth Planet (1966)