Ideally? The folks who're already showing the qualities of being moderators. "Reputation" and score are not a 'great' metric on their own.
I'd look at per-site meta participation. I'd look at their comments helping guide new users and the work they've done in and around the community. Essentially meta is our vehicle for supporting users, resolving issues, and influencing micro and macro decision making. We're going to deal with small issues (like advice on closed issues), difficult issues (irate users) and big issues (per site policy) and the quality of one's answers and engagement with the community as a regular user ought to inform how one would use these tools as a moderator.
While I've written in depth (or girth) here - quite specifically for a prospective moderator...
I would try to find the person I feel cares for the community the most, and commands respect, not by position, but by their actions.
The sum total of what they have contributed is more important than reputation, or election time activity. Pick the person who you feel is already doing the most good, and can use their additional moderatoral powers as a force multiplier to keep doing that.