The best opening line is one created by you and about you...
Have you ever noticed how dogs respond to the way we talk to them more strongly than they respond to the words we use when talking to them? That's because they don't know many words, they know sounds and they know intonation and they know volume. They like nice sounds and gentle voices, and they like biscuits of course.
When you speak to your dog at fever pitch it doesn't actually matter what you are saying, it really does not. All you are doing is frightening your dog and anyone else that is around at the time.
Contrast that to when you are gentle and respectful of your dog. Again it really doesn't matter what you are saying as the words don't count. Your dog is responding to the style and intonation and gentleness of your voice. Or to the eagerness of your voice when you are trying to persuade him to do something. At no time does shouting or talking fast get the desired result.
Dogs are smart. Smarter than people much of the time. So learn something here. If you want anyone's attention try not to frighten them or upset them with your loud, booming voice. Keep a lid on the volume and talk steadily and gently raising your voice towards the end of the sentence if you are looking for agreement or simply trying to persuade someone to your way of thinking.
This method of communication works out well and it works out consistently. Your dog knows this.
The point of this post is to give you the best opening line and in case it's not already clear here is my summary of what that is : The best line you can use is your own words, short and precise, maybe two sentences ending with a question mark. The professionalism is in the delivery and tonal approach, which should be in such a gentle way that even your dog would smile, understand and respect you. Don't overwhelm your prospect (or your dog) with loudness, jargon, or a series of requests. Just one request couched in one sentence and a call to action from your prospect (or dog).
The detail of this tip is not so much in what you say, though that is vitally important, but in how you say it, how long it takes you to say it, your tone of voice and your intonation at the end of the second sentence which needs to rise up sweetly in expectation of a good response. Practice this daily in front of a mirror as an actor would. The results usually include far better communication with your potential client, every single time. A by-product of this method is that even if they don't feel they have a requirement from you it will be hard for them to be totally dismissive which is what none of us like.