Should I hire the Hanahan agent who has already sold the most units in my building?

Selling volume in a specific building is worth more than a citywide ranking, since that agent already knows the HOA, the floor plans, and which units in your Hanahan building have sold for what. That kind of direct comparable knowledge is genuinely useful. It is still worth asking how that agent will represent your specific goals rather than assuming volume alone guarantees the best outcome for you.