Grams
Potatoes are sold by the kilogram.
You would need a metric calibrated scale to measure kilograms.
Kilogram
Any of them really. But most would use the kilogram
The SI unit for mass is the kilogram (kg)
The is only one Metric System. That is the one you would use. The units you use to measure mass is the kilogram (Kg).
The SI unit of mass is kilogram (kg).
Length: meter Mass: kilogram Volume: cubic meter
For an ant farm, I would suggest a kilogram. For an individual ant, a milligram.
Neither of those units will produce a very convenient number. But of the two, the kilogram is the better choice.
The metric unit for measuring human weight is actually the Newton (N). Newton's are a measure of force and weight is the product of the acceleration of gravity and an objects mass. The metric unit for measuring human mass is the kilogram (kg).
1 kilogram is a little more than 2 pounds.