for me it is the late Alex Sadkin who went from engineer to producer

I like the Vancouver connection of Bob Rock and Bruce Fairbairn (RIP) but I wouldn't say either is/was under-rated. 
 
Keeping it in Canada, not many people know who Murray Pulver is but his work with The Brothers Landreth, Sierra Noble and several others is getting noticed in the industry.

Max Bialystock and Leo Bloom