This is a hard question to answer – do we count the plant that there is the most individual plants of, or do we could the plant that there is the most mass of. This is because there will be lots of really small plants, but only a few really big plants.
You could say that moss or algae might be the most common plant in the world, because it is found all over in areas that other plants can’t grow.
Other people suggest grass or bracken fern might be the most common plant around.
It is almost impossible to work out, because different plants live in different habitats all over the world. By the time you had counted all the plants in the world, the place you started will have grown so much that the numbers are completely different!