The best? As many of the fastest processors (some motherboards support multiple processors). As much DDR4 or EEC memory as you can. The fastest SSD drive for OS and a couple more for storage and caching. The biggest, highest res HD monitors (x2).
A supported GPU.
Most efficient cooling...ect
You could easily drop $30,000 if you wanted to.
Maybe explain what type of work you do and how much you want to spend.
Guttrt-Fish is right. All depends on what you need, how much money you have. The best and shortest answer is: buy the best computer for the most money that you can spend. If you tell us how much money you wish to spend and what kind of work you will do and mabe we will be able to tell you what parts you should buy.
If you have unlimited cash just buy newest and best parts there are.