Best friends have each others' backs. While this relationship usually consists of two people, it can consist of a wider group of besties. This kind of friendship can look different for men and women, but themes of loyalty, respect, and enjoying each other's company are largely universal.

Below are a wide range of dynamics that may accompany this relationship. Use the ideas that suit your story and work best for your characters to bring about and/or resolve the necessary conflict.

Spending as much time together as possible 
Being each other's wingman (or woman)
Talking often via phone or text
Calling the best friend first when advice is needed
Sharing personal milestones (celebrating birthdays together, going to a kid's graduation, etc.)
Knowing the important people in each other's lives (kids, spouses, etc.)
Actively supporting each other in times of trouble (when one person hospitalized, the death of a parent, etc.)
Sharing the same interests or hobbies
Sending random emails or texts
Defending each other in the face of criticism or attack
Knowing each other intimately
Being able to be honest and shoot straight with each other
Having a history together
Not talking often but being able to pick up right where they left off (if the relationship is long-distance, for instance)
Avoiding topics on which the two don't see eye to eye
Having an affirming relationship where nothing unpleasant or difficult is discussed

A significant life change for one party that takes them down a different path (getting married, having a baby, changing careers, etc.)
One party moving far away
Someone experiencing a change in values (due to a religious conversion, personal growth, cultural influences, etc.)
A friend becoming involved in a new hobby, interest, or job that takes a lot of time
One friend joining a new social group (at their kid's school, at work, etc.)
Both parties becoming romantically interested in the same person
One person discovering a secret the other has been keeping
A betrayal