Maybe not the absolute biggest sleeper (I don't have instant recall), but a definite contender for the honour is Braid Hills #1 in Edinburgh - anyone who loves Painswick simply has to play it, because it's the course Painswick wishes it was. I'll also second the nomination of Longniddry. Reay, on the north coast of Scotland, is worth a vote or two as well, as is Dunaverty, near Machrihanish. (Tain, on the other hand, is not IMHO, but I know it's a course to divide opinions...)
Of American courses, I'd nominate Shinnecock Hills, simply because I played it back in 1993 - having had only vague memories of the Ray Floyd Open in '86 and not really knowing what to expect - and was utterly blown away. Shenecossett, in Connecticut, is another candidate on a different plane.
In Australia, I'd nominate Portsea. (Chris Kane hadn't oversold it before he introduced me to it...wow.)
Cheers,
Darren