What's the most important thing a business needs?

In digital marketing, a business needs a website above all things. Site needs no have some standards not only in its design but also page speed, compatibility with mobile devices and original, relevant content. But having all this without SEO (search engine optimization) is like having a car without gas. SEO also needs to be articulated with other strategies like social media marketing, paid advertising like Google Ads, back linking and many other. 
I have 10 + years of experience working with all size businesses. None of them could've really made an impact in their industries, grow or even sustain themselves without at least one of these tools.

I think a business will grow best with honest and humble leader(s).  Knowing what your competition offers and being able to clearly speak to your own benefits will help immensely.  Being creative with marketing techniques and teaming with others who can help you tap into your ideal market is key as well.

One danger I see in my work is a focus on releasing immediate discomfort at the cost of sacrificing sustainability. Leads, sales, profit, cash flow, etc. are all short term essentials. They may get you through the next quarter but, without guiding principles, when confronted with challenging times, your business will enter a death spiral. 
To be sustainable you need:
-- A clear reason why you exist
-- To know what behaviors you want to be known for and aspire to (behavior reveals your values)
--  To define core concepts (around 3) that will guide your decisionmaking
-- Choose your immediate focus -What's most important right now (6 months)

  

Start with a Vision and Mission Statement. Without the two, you can get lost in many directions. 

The other answers are important. I will add FOCUS!

In reviewing the posts, I think everyone has a piece of the answer.  The best part of this type of discussions are items you may tend to think of in the first 10 seconds like a strong foundation, focus, value to name a few.  First items come to mind are clients, products/services, expertise and so on.  They all play a role and depend on each other.  You could have something amazing that no one knows about, or understands it, or is unsure how it will be supported or more importantly; how does it solve a problem for them.  If you are not solving a problem and not properly putting the proper message across to your potential clients, then what you are trying to sell is most likely not going work.  Make sure you are solving a problem and you know how to communicate it so your client can identify with it.   Hope that helps.

I believe the most important thing a business needs is leadership with a clear focus and relentless determination to succeed; having these, the rest will fall into place.

4 main elements everyday.  Prospecting, Marketing, Lead Follow-up, and Closing The Sale.

It needs an owner with a passion for the business. The rest will follow, but if the owner lacks passion he's not gong to care, and an owner who doesn't care is going to work against the business. You can learn the other skills or possibly hire someone to get everything else you need, but it has to start with passion.

Customers, customers, customers