This is a loaded question, as some folks swear by certain brands, or reputations. Many will tell you that "only PI machines are good enough for gold prospecting", but this is not necessarily true. PI machines go much deeper, less interference from mineralized soil, but don't have as much sensitivity for tiny gold. The following models have good reviews and user's comments. Check them out . Kevin Hoagland reviews many of these on the detector prospector forum.
Right now, the hottest detector for gold seems to be the Minelab Gold Monster 1000. It is a PI It usually sells for less than $900 USD. Several higher-priced Minelab detectors ($2,500-$9,000) are popular in Australia, if you are thinking of going all-out.
The Fisher Gold Bug II is an "old standby", an older VLF machine (71KHz) that has found A LOT of gold,, sub-gram pickers at 3"-4" Usually priced around $650-$700
Nokta/Makro are sister companies, with separate model lines. Nokta's AU Gold Finder is much like the GBII (analog, no display), and the price is about the same. Operates at 56 KHz, so will find fine gold. It has a waterproof coil, but the box is "water resistant", but can be belt- or chest-mounted.
Makro's Gold Kruzer operates at 61 KHz, is completely waterproof, and has wireless headphones. Digital controls, and LCD display. Price is comparable to the Nokta model.
White's GMT 24K is White's legendary "Goldmaster" model, upgraded and tweaked for 21st Century. Many swear by White's quality and reputation. Operates at 48Khz, and has waterproof coil, but entire detector is NOT submersible. Priced about $750.
Garrett's AT Gold is a favorite of many, and many swear by it as being "the Best", but it is pretty old technology and operates at a lower 18kH.