There are countless philosophies of life. But since the question asks for ONE, below is one I think everyone should hold to their hearts. “Do unto others as you would have them do unto you” or any variation of it. It's simple and easy to understand.

A meaningful life philosophy is to see life as something happening within you, rather than outside you. Then you can seek self change rather than fight windmills, and try to correct the world around you. As the famous saying goes, “be the change you want to see in the world”.

Quite literally, the term "philosophy" means, "love of wisdom." In a broad sense, philosophy is an activity people undertake when they seek to understand fundamental truths about themselves, the world in which they live, and their relationships to the world and to each other.