@Absinthe SBCL. It is actively developed (unlike clozurecl), with monthly releases. It compiles to native code (unlike CLISP), the compiler has many unique features. That said, many implementations have their uses but if you're asking, then the answer is clear.

I very rarely hear about GCL. Last time I saw it mentioned, was in a source code hunk meant to be read by GCL specificially, followed by a comment like “my, is GCL broken”.