Since I worked as a kind of SDET on a tools team at the time my answer was 'developers'. Then the interviewer dove deep to find how I interacted with my customers. I had some examples where tools had been broken for developers but I didn't realize it mattered. I discussed how I might improve communication with my customers to get them complain loudly when something is causing them pain. I actually think this is one of the best interview questions I have ever heard of.