It’s one which is most scarce – TIME – and the successful know that

It cannot be created or borrowed

Every person has 24 hours, and once they pass, they will NEVER RETURN

The trick is utilize your time in the best possible place you can

Ideally we should do stuff only which we MUST do ourselves

DELEGATE anything else that can be done by someone else

This will take disciple, planning and a burning desire to head towards your goals

PS.
This is important for testers and software development teams too

References:
“The art of self-discipline” by Brian Tracy
“The Millionaire booklet” by Grant Cardone