A good friend is one that wants to spend time with you and be there for you. You’ll likely have many people in your lifetime that will fit this description.
A great friend is one that wants to spend time with you and be there for you as often as you would like to be there for them. These will be a much smaller number of people, perhaps a handful, over your lifetime.
The difference is nuanced, yet strikingly important and not easily measured quantitatively.
Choose wisely where you invest the best of your time and energy.