What is the best watch in the world ?

No one watch qualifies as best. It depends on an individual's preferences. Broadly watches can be classified into Digital and Mechanical watches. Mechanical watches can be further categorized into automatic and non-automatic. 

Digital watches typically use an electronic circuit to keep track of time. They generally are mass-produced and are inexpensive but highly accurate in keeping time. 

On the other hand, Mechanical Automatic watches typically use a rotor, or metal weight, to power the timepiece and keep time. The rotor is powered by the movement of the watch wearer's hand. Premium watches are handmade and can be pretty expensive. They also are the least accurate in keeping time. However, watch enthusiasts still love and collect these watches. 

Mechanical non-automatic watches typically use a quartz crystal to keep track of time. These are inexpensive and very accurate at keeping time. 

So the best watch for you depends on what you are looking for. If you have a tight budget and care about accurate timing, then digital or non-automatic watches might work well. On the other hand, if you are a watch enthusiast, then Automatic watches are a great buy.