The best metal detector

For a beach or water hunter the best metal detector to buy is the one you choose after doing research on metal detectors. 
Buying a metal detector just because someone you see uses one is probably the worst thing you can do, unless that person is well known for their metal detecting finds and they have used the metal detector for quite some time.
Even then I recommend heading to your local dealer to see if you can hold the same metal detector in your hands.
Its no secret what make and models of metal detector I prefer to use, but I often try other brands just in case something is better. 
So far I have not used anything that would make me want to change from using my favorite Minelab metal detectors for my style of beach and water hunting. 
Actually, I have to slow my roll on trying other brands as it is become expensive losing money selling metal detectors and search coils I tried, tested and decided I preferred what I have.
Research before you buy any type of metal detecting equipment is very important, buy something because you believe it fits your style of hunting and go from there.
For example, perhaps a non waterproof but more versatile metal detector is a better fit if you are a beach hunter.
If you intend to only search inside the water, its either going to be a waterproof VLF or pulse induction metal detector choice for you.
The more things on a metal detector that aid your type of hunting the better.
For example, it makes no sense hunting in the all metal search mode digging every metal object using a top of the ling discriminating VLF metal detector, although plenty of people do.
Buy a deeper detecting pulse induction metal detector if you want to dig every piece of metal.
The same applies to pulse hunters who discriminate by target response, why not buy a discriminating VLF if you are going to reject targets in all metal.
If you have to turn a metal detector into something resembling a metal detector already out there, save up and buy the darn thing you are trying to copy.
Heck it may actually be a cheaper metal detector and a better fit for your style of hunting. 
A high priced metal detector does not guarantee more or better finds, unless it is the right fit for you.
Look at the important things that make a difference between metal detectors, ease of use, controls you can adjust to suit different situations and can it detect the type of targets you are searching for. 
Certain metal detectors are hot on small gold, others optimized towards detecting silver and some metal detectors are better suited to detecting large iron targets.
All of these reasons are why I tell people to do lots of research before buying a metal detector, I also always recommend buying the metal detecting equipment you are most comfortable using.
A metal detector should be a tool used to help you detect what you are searching for.
I doubt you would buy a tool at the local hardware store that was not suitable for the job you intended to use it on.
You would buy the right tool for the job, buying a metal detector or any type of metal detecting equipment should be the same.
Start by identifying the targets you are likely to detect in the areas you search, then research what metal detector is regarded as being the best at detecting what you are searching for.
Pull the trigger on that metal detector, read the manual several times and commit to it as that metal detector will be the one you compare all others to in the future.
Research and choosing the right tool for the job are the keys to getting it right when choosing the best metal detector.