I use Common Lisp for its really insane metaprogrammability (reader macros, compiler macros, ordinary macros, and the MOP to edit the object system) and the abi

I use Common Lisp for its really insane metaprogrammability (reader macros, compiler macros, ordinary macros, and the MOP to edit the object system) and the ability to bend to the problem that I am trying to solve. It has decent interoperability via its CFFI interface.