What is your favorite code metric?

You don't need to run any build or external tool to get this. It is always there when you code and changes actively as your code evolves. What would it be?

My most favorite is obviously "Line Numbers".

Line numbers are available in all IDEs but mostly not actively used as a code metric during coding. Switch them on and notice them actively when you code, they tend to have an interesting effect. Probably the classes and functions will be shorter, less bloated and concise. Based on personal experience :)

What is your simple metric to improve code?