Gucci Nobile was an easy choice, however, with respect, I don't really think that Gucci Nobile qualifies as a "sleeper".
Most Gucci bashing threads (of which there are many), make fond reference to Gucci Nobile, so it has never been forgotten.
Helmut Lang Cuiron was discontinued some time ago. It was a Basenotes darling for a long time thereafter.
I've noticed very few references to Cuiron in recent times, so perhaps it now qualifies as a "sleeper"... but even this is questionable...