Every startup has three customers, but this is something which lots of startup founders don't seem to realize.
The most important customer is of course the person who's going to pay you for using your product or service. This should be your primary focus . If you can delight him, 90% of your battle is won. However, that's not enough.
It's equally important for you to treat your investors as customers as well, because you need to keep them happy as well . You will raise more money in the future, and this is true even if your company is doing very well , because you need more funds in order to scale up and to grow. Your existing investors will be happy to give you more money if you treat them well, and they will help you to attract new investors too. You do need to lavish time , energy and love on your investors ! It's a small ecosystem, and getting an investor-friendly reputation will help your company to grow.
Finally, the heart of your company are your employees, and you are only as good as your team You need to be able to sell your company to prospective employees , and convince A-graders that you are the right choice for them. This can actually be a very hard sell, because it's hard to convince the right people to take such a big risk , given the fact that you are cash-strapped and there is no job security in a startup.
As an investor, this is a very valuable metric for me to track, because they will do far more due diligence than I ever will, because they have so much more at stake ! Can the founder attract experienced people to come and join his company ? Are they willing to give up a cushy job with a fat salary to work for him ? Can he sell his dream to them ? This shows that he's capable of creating a company with the right culture, and I much happier to back him.
If you do need to select the between the three, then put your employees first. This might seem counter-intuitive, but it's your team which creates the product which delights your paying customers - and if you have paying customers, your investors will lever have any reason to complain. This is why the mantra secret to running a successful startup is to Put Employees First. Everything else will fall into place automatically !