If you're looking for the "power" in PowerPivot, these common DAX functions highlighted by expert resources like Microsoft Support and Analytics Vidhya are essential:
: It bridges the gap between different data worlds, including appendixes for SQL, MDX, and DMX query design.
: It contains over 250 downloadable DAX formulas that cover everything from basic filtering to complex statistical and time-intelligence functions.
RANKX : Automatically ranks items (like products or sales reps) based on a specific value. Practical PowerPivot & DAX Formulas for Excel 2010
In his book author Art Tennick provides a comprehensive guide for extracting actionable insights from massive corporate datasets. This "hands-on" resource is particularly noted for:
DATEADD : Used for time intelligence, such as calculating "Year-Over-Year" growth by shifting dates.
CALCULATE : The most powerful function, used to modify the filter context of a calculation.