Children of Men - Grabbed me by the balls in the theater.
No Country for Old Men - The narration followed by our introduction to Anton Chigurh. It's just so well done.
8 1/2 - A visually excellent sequence that introduces us to the mind of Guido.
Narc - Grabs you by the balls.
Sunset Blvd - Iconic, simple, just perfect.
Dr. Strangelove - The opening titles set the entire tone of the film. The playful symbolism, the handwritten titles, the music...
And I think this opening is just a lot of fun: