Hmm.... there are a lot of great opening moments in movies. I'd say a tie between these for me:
Jaws... Raiders... X-men 2... Star Wars (before they got numbers)...
I'm thinking 'grab you' openings of course.
My two personal favourites are:
Blade Runner and 2001 (in a cinema with a very large screen).
Try to be reasonable... its easier than you think...