Chuck nailed it for me. There is no one single most important aspect of running a business.
This is probably the biggest problem a business owner faces: Everything is important.
Profitability is essential if the business is going to have enough operation capital. Customer satisfaction is essential to the building of a good reputation. Good people skills are paramount in dealing with both customers and employees, not to mention the army of inspectors and agency representatives, any of whom have the power to make your life miserable should they feel offended or not sufficiently respected.
Then there are management and planning abilities, creative vision, skill, and a huge list of other factors, any of which carry the potential to pull the rug out from under the success of your business.
The old saying about being a jack-of-all-trades and master of none cannot be applied here. You need to be a jack-of-all-trades and master of all!