Dplyr (and its spin-off dbplyr) is to me a fantastic practical example of the power of lispy metaprogramming. While R gets knocked a lot for not being a "real" programming language, or being weird, or just generally being different than e.g. Python, I don't know that I've seen a cooler example of high-level programming language ideas expressed as cleanly.

Ditto the rest of the tidyverse.