No successful treasure hunter that I know of relies on just one instrument or technique to locate a target because nothing is foolproof. So typically the treasure hunter may use dowsing, an aura camera and other long range locators such as a multi frequency discriminator (MFD), an earth field generator or even a sniffer dog, before checking out the consensus target(s) with a metal detector.
When it comes to metal detectors then there are a number of factors to take into account, particularly the size, composition and depth of the target. In Britain, many of our caches have been broken up and scattered by centuries of plowing, so I have been on a mission to find a detector that can detect very deep on a single coin. For intact caches on inland sites, the low cost solution is a two-box detector but these are said not to work on beach sites, where I have an interest in recovering wreck, so I have a Detech pulse induction detector with a range of coils up to one metre square. Recently a new range of detectors, like OKM, have emerged with video screens and impressive claims, however, until I see one in action, I really cannot comment on how effective they are.