It depends, I think, on how one defines "stability". If we define it as "the tendency to go where you last told it to go" I'll put a Pitts up against anything you want to name. If we define it as "the tendency to return to straight and level flight" then the Pitts won't even show up to watch that competition, much less participate.

Perhaps, instead, we define it as "the lack of nasty surprises"...say, the tendency (or lack of it) to snap in certain situations. The Pitts can be a snappy little freak....I say frequently about mine that it's "snap rate" is greater than its roll rate"...but at the same time, my Giant Aeromaster won't snap if you hang a cinder block from one wing. *shrug*

Again...it really all gets back to design, intent, and build....and, of course, the pilot.