Personally, I will give my top 3:
Shawshank Redemption: Morgan Freeman has to be in the discussion of course, his voice is gold and his delivery likewise. He strikes a good mix between humorous and serious, and towards the end his "Get busy living, or get busy dying" epilogue is a great way to end the film.
Apocalypse Now- Martin Sheen's narration gives you the feeling of being in the mind of a Vietnam special forces soldier who's journeying into the depths of human insanity. Amazing job.
Goodfellas- When listening to the narration of Goodfellas it feels like you're listening to the real Henry Hill tell you his life story and fall from grace, it flows so naturally.