Common Lisp's format function.

Because it supports some crazy convenient formatting directives, such as: writing numbers as words, writing numbers as Roman numerals, correctly writing plurals, etc

The code in the image will print:
one cat
two cats
three cats
...
seven cats

That's hot. ++ for you and Common Lisp.

@Letmecode common lisp is fairly paradigm agnostic. It's not as FP centred as scheme

@Letmecode there are more ways of dealing with data structures mutably than immutably in CL, and the destructive functions aren't very clearly labeled as being destructive. Scheme handles this much nicer by how all its destructive functions are suffixed with a !
That said, I prefer a lot of things in CL, enough so that I can look past the somewhat dated keywords