Best openings (off the top of my head) have got to be...
Star Wars: Episode IV - A New Hope
The scene that introduces to cinema's greatest franchise. Not many movies would throw the audience into a space battle immediately, but Star Wars does it with style. Oh, and the reveal of the HUGE Star Destroyer is so iconic
Casino Royale
The film noir-esque black and white opening, the calm and intimidating interrogation, the high octane brawl in the bathroom, his first two kills, the Gun Barrel (turn, aim, BANG)... 007 is re-born
Jaws
A few minutes of terror created by incredible cinematography, music, and editing... and the suggestion of something lurking below the surface
Raiders of the Lost Ark
The calm, cool, and collected draw back and crack of the now famous whip, the image of the equally famous fedora silhouette, and the sun light revealing the face of cinema's greatest adventurer