I have to agree with Kadrina. Mods shouldn't be chosen just by activity. Just because somebody is active or popular, doesn't mean that they're going to be responsible in a position of power. I've seen people who seriously shouldn't represent anyone as mods cussing at rule-breakers in public, treating users with disrespect, creating their own idea of how to follow the rules, and being generally rude and unprofessional.
Mods should be chosen out of some sort of verifiable moderation experience in forums and demonstrate that they can remain professional and act within sound judgement under stress. This is a really big site. There are going to be a LOT of problems. Moderation is a job first and foremost. People should be hired based on their ability to handle said job, not, as Kadrina said, in a popularity contest.