An experiment require 20 ml of water. Which piece of equipment would be the best choice to measure the water?

it depends on how accurate you have to be. if you want to be pretty accurate, then you can chose to use the 10 ml pipette even if you have to use it twice in exchange for accuracy. if accuracy is not extremely important, then use a 25 mL graduated cylinder. it is not ideal to use the 50 mL grad cyl because it's too big, while the 25 mL glass beaker has less graduations than the 25 mL grad cyl.