I think the best opening scene ever may be in "Ulysses's Gaze", with the director coming home and the excerpts from the first greek film.

I also love the openings of Dead Man, Johnny Depp in the train,
and
"When father was away on business" by Emir Kusturica, one of the most complex and multilayered scenes I've ever seen on film. It's in fact better than the whole rest of the film, though I like the whole very much.

Also to my mind comes 2001- A space Odyssey 8-)