It’s an important question, for a leader or for anyone. Usually the answer is You, Now. Otherwise you wouldn’t be wondering about it.
But sometimes the answer is Someone Else, and/or Later. And it’s important to remember that.
There’s a famous prayer that goes
Grant me the serenity to accept the things I cannot change,
The courage to change the things I can,
And the wisdom to know the difference.
That last line is also very important. Some things you think you can’t change, you can. Some things you think you can change, you can’t. And it’s important to know which is which.