For me, I'm going all the way back to 1968 and the opening sequence of 2001: A Space Odyssey. You have to put your mind back in 1968 in a 70mm state of the art *LARGE* theatre (not the boxes of today). This opener just blew people's minds up it was so different from anything else they'd seen on the big screen up to that point. I think the next film to even come close to that was the very first Star Wars when it was released in 1977.
And one of my favorite openers to one of my favorite offbeat films of all time: Kelly's Heroes.