SBCL is an implementation I love working with because updates are steady and the software is stable.

But the real superpower, in my opinion, is that, because the compiler and standard library are written in Common Lisp, you can reach in into the internals of SBCL for your own projects—as if SBCL were just another Lisp library. Is it advised to use unsupported APIs? Definitely not. But it's nice to be able to have seamless access to the same facilities and optimization tools (e.g., DEFTRANSFORM, DEFINE-VOP) that the SBCL uses for its own implementation. You can build impressively clear and highly efficient code this way, essentially by extending the compiler "in userspace".