This question causes a hot debate in the world of business.
My answer is simple; they are all the most important. If one of these functions in a business is weak, it could significantly damage success and ultimately without one of the functions the business wouldn’t remain for long.
Each department is fundamental for a successful business. The operations team create the product, the sales/marketing team generate publicity and sell the product, finance ensure the business stays profitable and human resources ensure the employees are motivated. Well that’s the theoretical view of a business at a basic level. In practice, a business exhibiting all of these functions perfectly is very rare, and would be a recipe for mighty success.
Ensuring all departments of the business work in tandem should be the main focus.
Many businesses around the world seek this focus, and inevitably have become global brands.
Any comments on what view you pose would be much welcomed.