`dplyr`, the data manipulation library in R.

Some prefer base R or data table, or python pandas, but for me, dplyr is essentially a _perfect_ data manipulation library for small to medium data, and nothing comes close.

Given that for a data scientist, data cleaning is “80% of the work”, and in a world where science is increasingly data science, means that dplyr has done a LOT of good.

Honorable mention goes to ggplot2, the plotting library, and the rest of the tidyverse.