Pounds or Kilograms
For a desk like one used in schools the best unit would be pounds. Good luck on your measuring!
unit for measuring length
The best unit is a litre.
The best unit for measuring the length of a pencil is inches.
Mercury is usually measured in grams since it is usually used in quantities where the magnitude is best measured in grams. It really depends on what you are using it for as to what unit of weight is best suited for measuring it. It obviously can be measured in any unit of weight.