You can locate the most engaged subscribers in your audience using custom segments (under Audience → Segmentation).
For example, you could create a segment of all readers with an open rate greater than or equal to 80% and send them a special loyalty reward.
More segmentation tips can be found in the following articles and videos:
- How do I create segments?
- Intro to Segmentation Part 1
- Intro to Segmentation Part 2
- Create Segment from List Upload
- Segmentation with Custom Fields