thank your for your reply. Well, I think what you are stating in your intro goes without saying and it might be useful to state that in the interview for the sake of completeness.
However, if the interviewer insist that you choose one KPI/ratio only, without clarification on the industry, the goal, whatsoever, which one would you (or the community) pick?
Again, I think the EVA is one of the KPIs that you could justify, as it takes into account the regular business activities/operations (via NOPAT) and accounts for the capital structure to a certain extent. One might even go as far and state, that it includes measures of productivity.
Happy to discuss further!