I've been through the whole gamut of PUA opening material. For the most part, too much stress is placed on the opening, the real work is what comes -after-.
But the PRIMARY factor in the opening is comfort and proximity. Are you positioning yourself in an AWKWARD social situation.... Or, does she feel at ease when you speak to her? Secondly, is she naturally in your 'space', so it makes sense that you're speaking to her, or did you basically follow her, or track her down to find her like some kind of freak?
Awkward: You walk straight for a girl you don't know, ask her something incongruent or forced. Likelihood of rejection is VERY high. All her alarm bells are going off. She is thinking "What does this guy want?" or "Is he hitting on me?"
Awkward: You introduce yourself to someone in a forward manner.... out in 'daygame', at a coffee shop, etc. This is quite socially strange.
Smooth: You look at a girl who is ALREADY standing near you. You use a basic but socially 'acceptable' opener, such as "Hey what's the occassion tonight?" Nothing fancy, just get 'in the door'. The real challenge is what you say next, keeping the conversation rolling, making banter, and most importantly projecting a 'sexy vibe'.
That's right, your THOUGHTS are written on your face. The slightest bit of low self esteem... girls can sense! The more congruent you are to feeling good about yourself, the more your success will sky rocket. So whenever opening, remember the REAL secret ingredient... which is vibe!