Find out who you talk to the most
To find out who your best friends on WhatsApp are, if you have an iOS device, head to Settings -> Account -> Storage Usage. Here you'll see a list of the people and groups you message on WhatsApp along with the total number of interactions you've had with that person/group.
If you select "Size" at the bottom right of the screen you can see who you have spent the most data WhatsApp-ing.