What is the one thing I should focus the majority of my time in my startup?

Revenue solves all problems.

If you want me to be more technical, cash-flow.

I would say definitely focus on customer satisfaction which includes building better products, giving better customer service and providing better user experience.

You can find customers or sell products until you have a great story...

To echo Dan and Chris' sentiments, customers and cash-flow are your most important goals. That said, I would strongly suggest you spend more time focusing on customer development than cash-flow, as a business without cash-flow can survive with investment, but a business without customers is unlikely to survive at all.

As an excellent resource, I'd suggest you take a look at The Entrepreneur's Guide to Customer Development, available at http://www.custdev.com/ (I'm not affiliated in any way, just a fan).

Of course, if you'd like to chat more about startup focus, I'm happy to setup a call to discuss tactics I've used to build our customer base AND revenues by more than 300% year over year.

If you should focus on only one thing, it is not your startup (you need partners/co-founders).
You can allow yourself to focus on one thing only if you properly delegate other aspects... and if you do this, you should already know what to focus on.

(Before I start, I am in retail, so my answer comes from that perspective).
Success comes from all the usual - good business model, great product, strong business foundation, working capital, etc. But, the one thing that is hard to control is "buzz". You of course must do all the obvious things as listed above, things you probably know, but getting a product noticed in a crowded marketplace is virtually impossible.

Of course, everything depends on your goals and what you want to make of this business - do you want a 500K business, or a 500M business. But, if you can separate your product from the rest by having a real edge, something very different and marketable (not just that you think it is, or people around you who like you tell you it is), or you can connect with another company or individual or celebrity who can give you a leg up.

Other than that, hard work, integrity and building a team of great people, and then applying liberal amounts of patience, will give you the best chance of long term success.

I often get called by companies when their first version of a product isn't exploding and they think marketing will fix it. But nothing spreads the word about a bad product better than good marketing. I will actively refuse/postpone clients who aren't ready for us yet, because marketing cannot fix a product/market fit problem. ( We can still sometimes help--you might need your google analytics installs retooled to get the right data, or to get the most out of the reports, some insights about what is and what is not working, or to create a plan for testing.)

So, my advice to you is to build a product that doesn't require you to spend money on marketing. Understand and love your customers so that you can anticipate their needs. Stay focused on it -iterating, evolving, measuring, growing-until you have very low churn rates. Low churn/recurring revenue will be what allows you to spend money on marketing and possibly raise investment. If you spend money finding customers who don't stay, you're only renting them, not acquiring them.

Growth: Distribution growth, customer growth.
And how you get it? From my POV something very important: REPUTATION. I would say concentrate on making a good reputation of your company. The growth will come from that.
hope it helps.