Regardless of what type of organisation you work for and what type of role you have, the ability to use Microsoft Excel well makes a huge difference to both the ease and proficiency with which you are able to perform. I was recently amused to see that even the security guards were using Excel in the ID card office - pretty sure that it was not part of their job description when they signed up!
It is easy to spot someone who is an Excel master. For one, he or she never uses the mouse - it is all about keystrokes - navigating within a worksheet, tabbing between work sheets and work-books all through key strokes that can be keyed in very fast. Another clear differentiator is the logic that masters use. From stating overall objectives of the analysis, to laying out contents, to structuring key inputs and noting key assumptions to an efficient calculation engine and finally an easy to understand set of outputs. This kind of mastery comes with months if not years of daily interaction that, admittedly, may not be the norm in many jobs.
However, the 80-20 rule applies in spreadsheet mastery as in all other things too. In addition to the basic functionality of spreadsheets that allows you to build tables of data based on simple arithmetic functions such as add, subtract, multiply, divide - there are three functions in Excel that I would argue are the most widely used and the most useful. They are the IF function, the VLOOKUP function and PIVOT TABLES.