Here in Australia, I think the #1 detector for serious treasure-hunting is probably the Minelab CTX 3030. But If you're just looking for maximum depth rather than versatile discrimination, target ID, etc., you should probably consider one of Minelab's GPX series of PI detectors, either the GPX 4500, 4800 or 5000. There's a big range of coils available for them, from Coiltek and NuggetFinder as well as Minelab, but these are primarily gold hunting detectors, so there's no worthwhile discrimination and you'll likely find plenty of very deep trash as well as treasure.