Desperado

The bar shootout in Desperado is not only the best action scene in the film, but also one of the best gunfights ever made. This is, to say the least, problematic. 

While the rest of the flick is decently interesting -- Salma Hayek's body double gets nekkid -- the film eventually collapses under the weight of its own action. At the film's climax, we aren't even shown the brutal gunfight between the Mariachi and Bucho's men: given how cool the very first gunfight is, the audience already understands that the Mariachi probably kills everyone and makes it out alive. It's a pretty unusual ending, but not a particularly satisfying one. Desperado opens with its greatest gunfight and continues on with action that gets progressively less and less intense.

You're probably better off just watching this gunfight, then popping in Once Upon a Time in Mexico.

Swordfish

Swordfish, unlike Desperado,  is not an otherwise enjoyable film which happens to be overshadowed by its great opening scene. Swordfish is a bad film (the kind that treats hacking like some sort of tech-driven form of sex), and its incredibly badass opening scene only exists to distract us from that fact.

Suspenseful, disturbing, and visually spectacular, the opening to Swordfish is everything the rest of the movie isn't. In a way, this makes the opening scene a relief; after watching the above video, one can live content that you really don't ever need to watch the rest of the movie.

Way of the Gun 

The last gunfight is admittedly pretty damned cool, but you just can't top a profanity-spewing Sarah Silverman getting her face punched in for sheer badassity. 

This opening, which has absolutely nothing to do with the film's plot other than showing Parker and Longbaugh to be total badasses, is one of the few outwardly hilarious moments in Christopher McQuarrie's otherwise straightfaced crime flick.

The rest of the film is good enough that it's still worth watching even after Sarah Silverman walks offscreen with blood streaming down her face ("You look beautiful! More beautiful than ever!"), but the movie never manages to truly outdo its totally pointless, totally kickass opening scene.

Hard Candy

Hard Candy's disturbing, unusual, provocative opening scenes serve two purposes, one intentional and the other not. Firstly, the viewer is meant to feel disturbed, concerned, and conflicted as the wide-eyed Ellen Page seduces and is seduced by Patrick Wilson; this confusion serves to make the film's big first-act twist (surprise -- it's a revenge flick!) even more shocking. The unintentional, but far more prescient side effect of this is that the rest of the film is never as confusing, disturbing, or downright unusual as it was during its first fifteen minutes.

A revenge film is pretty simple for an audience to digest, even if it comes equipped with some baggage about the nature of morality and character identification. Flirtation between a 15-year-old and a 30-year-old, however, is a hell of a lot harder for an audience to pin down or deal with. Thus making it infinitely more interesting to watch.

Saving Private Ryan

Spielberg's Saving Private Ryan can be accused of many things: sentimentality, hypocrisy, heavy-handedness. None of these complaints, however, can be accurately aimed at the film's horrifying Omaha Beach opening. 

Spielberg spends almost three hours convincing the audience that "War is Hell," but he really needn't have gone beyond this terrifying opening scene, which says all that really needs to be said. Without the hackneyed dialogue or overtly coincidental plotting, the massacre of hundreds of Allied soldiers without rhyme or reason really informs the audience as to what war is really like.

Ideally, one would watch the Omaha Beach scene, then leave before Tom Hanks can eventually be shot by the one Nazi he lets go free, or at least before the sappy "tell me I've lived a good life" epilogue.