The find an agent section is automatically sorted by the system showing the agent with the most properties in an area.  The top 3 agents are automatically marked in orange and are ticked.

For example, at the time this was written, in Belfast the agent with the most properties is UPS (Cavehill Branch), as per the screenshot below:

However, if you click on the tab for Student Rentals, then R Stanley Laird is at the top of the listings as they have the most properties:

As a little tip, if you type Ctrl & d on your keyboard, the number of properties for each agent in the category will show on the page.