What is your best email subject line that actually gets people to respond ?

I have seen several things work well to get people to open and read the content but getting them to respond is a different subject.  The subject line is much different than the content which is even more important. There is no such thing as a "best subject line" "You need to throw a bunch of mud against the wall to get a little to stick" .
The goal of the "subject line" is to get them to open and read the content. If they don't open they won't read.
When I know my target audience and they know me I usually just use a subject that relates to them " Don't miss the next event" or "More information you requested" or "I hope we see you again on (date)".
If I don't know them and I just want to be sure they open and read the content I may use something like "This may be what you need", "5 reasons you should NOT attend", "Are you in?", "Here is your personal invitation" or something that will create curiosity or interest in opening. 
We are so saturated with incoming emails that you need to be different in order to get them to open. Once they open the content needs to be compelling for a response.  

I've used this for 3 years and it works great!  4 simple words....

What can you afford?

I guess it would be easier to list what you cannot afford, right? An I.T., guy that never shows up, being hacked by your employees that open every email without thinking, passwords taped to your employees' computers, forgetting to remove permissions when an employee leaves, and the list goes on….  It helps them realize the value of your services and many end up asking for our free I.T assessment, which was the main purpose of the email.  It always gets read and it injects a dose of the bitter truth along with subtle humor.  This can easily be customized to fit any business.

Subject lines are used to get people to open emails. The subject body is used to get them to respond.

Here are some common mistakes I see all of the time. DON'T DO THIS:
-- Talk about yourself. Nobody cares. People care about your solutions to their problems so keep it short. 
-- Use jargon to talk about what you do. Confused minds always say no.
-- Pitch their services before they even know what problems I'm trying to solve. 
-- Send unrequested follow up emails. If you didn't generate enough interest the first time, there was a reason. 

Here's what to do instead: 

Email Subject Line 
-- Add 1 to 3 buzz words for niche
-- Write 5 different versions and study conversions

Email Body
-- Bring your opportunity to someone by researching what the customer actually wants. Online profiles make this easier than ever. 
-- Ask a question that connects with them
-- No more than 3 sentences
-- Guarantee results (ONLY if you can deliver)

Depends on the relationship with the recipient. If it's someone we're already doing business with, and it's critical, I use, "RETURN RECEIPT REQUESTED" followed by the rest of the subject line. Then they know to respond and it's serious.

I don't like games in my subject lines so I resort to concise words that indicate what the email communicates.

There are many I used and thank goodness for the platform program I use, as they tend to send what is called open rates as the goal of the "subject line" is to get them to open and read the content. If they don't open they won't read. So here is one I used that got everyone to open read and reply back, Subject line: Is the Love Gone? from there my body of my email was based of I haven't heard from you in awhile has the love gone away, and follow it up with to ways to coming back to many service I do.

The one that addresses their problem head on with the solution they want and followed up with an offer so compelling that they can't say no. If you don't address what they want they have no interest in reading on.

They don't care about the platitudes or features of your business. We are inundated with data each day. If you don't stand out above the white noise of everyone else, you won't get noticed and you won't get a response. 

It is easier done than said! :)

The best intro line is as follows:
Let me introduce myself and my name ————— and the company that I represent is——————- and we have in business for—- years. Let me introduce our company to you.

In Closing will it ok for me to give you a call to discuss your requirements.

action required

I have used the following line when someone to whom I sent an email doesn't answer me. It would be a person whom I know or I might not be very familiar with.

This is the subject line: "Did my email get lost in your inbox?" This worked really well. The person answered me quite quickly.

It depends on many variables (relationship with recipient, content, nature of email, etc.) but often I have found a question to be very engaging. You'll want to keep it short and leave a little mystery to keep it intriguing and make sure it is something the reader will want to respond to. Asking their opinion ("Which is best?") rather than something they may answer to themselves rather than respond like ("Are you prepared")