I would say "high performance customer". The one that is going to push your product to its limits, trying to make it do more than you thought it could. Those limits are where you need to add features so people don't have to hack around the limitations.
I would invoke the "skin in the game" metaphor. Listen to customers who have a vested interest, for their own reasons (brand building, networking, the "expert" part) as well as for optimizing some part of their professional workflow, making your service a catalyst of their moneymaking abilities (the benefitter part). The hacker part to me is mostly a consequence of frustration or necessity, and that is different for every industry.
If a publicly outspoken professional uses your software, engage them. That has been very helpful for our company every single time. It resulted in great publicity, brand building and in almost all cases meaningful insight into the what and why of their actual business problems.
