Who are your best customers?

Best can mean

1. most profitable - relatively easy to find the biggest spenders
2. most loyal - harder to quantify, but generally a better measure. If someone spends 100% of their overall budget with you, their trust in your product/service is undeniable. These are your advocates.
3. most beneficial - perhaps hardest to quantify, these are the customers who will bring most value to you in the long term, buying whatever you produce, sharing their purchases online and staying with you even when their circumstances change.

Each group has engaged and invested in your company. Your marketing needs to reassure them they made the right choice, keep them engaged and give them that feeling of exclusivity that makes frequent flyers clammer for the first class lounge.