The centimeter would probably be the most convenient in everyday use.
You could use any of them but you end up with very big or very little numbers which is not that convenient. Who want to know their 12cm pencil is 0.12m long, 0.000,12km long or 120,000um (microns) long.......
In scientific or engineering use the centimeter is a little frowned on because it is not an "official" part of the SI system of unit prefixes. If you are mixing in these circles you might be better using milimeters (but I mix in these circles and have never been asked to measure my pencil :-)
a ruler
which is a better estimate for length of a pencil
Millimeters
Centimeters
The centimetre.
2.5
Centimeters
Centimeters
length
length
The length of a pencil. your waist/chest measurement the dimensions of a table
Centimetres, millimetres or inches would be most appropriate for measuring the length of a ruler.
a ruler
Micrometer
a ruler!
which is a better estimate for length of a pencil
A ruler would do the trick.