What should be the most appropriate content for calligraphy works to give to friends?

Positive, inspirational, famous sayings and aphorisms will do.

Such as: the sea is wide with fish jumping, and the sky is high for birds to fly;

Mochow has no confidant in the road ahead, and no one in the world knows you!

There is a road to diligence in Shushan, and there is no limit to learning the sea.

Water the flowers, water the roots and make friends. Wait a minute.