[TurboHaskal]

He probably means Allegro CL which is in my experience the best one.

All the OS Lisp GCs are stuck in the 80s and honestly not very good. The one in LispWorks is pretty good though but I wouldn’t call it state of the art. It just works.

[fm2606]

How does one go about testing how good a GC is or isn't?

I'm most interested in SBCL and JVM.

[rurban]

I would call the LispWorks GC also superb, and it's even open source!