There are only three possible answers. I'll tell you, but they'll change the answer once I've told you, so as to fool you, but in a few years they'll admit what they did.
1. Chuck Norris. I don't think I have to explain this, what with the entry in the ChuckNorrisFacts page. The Chuck Norris unit will either be some unstoppable combat unit or will be some sort of peace-keeping unit (as in Walker Texas Ranger). There's a slight chance he'd be some sort of entertainer or missionary, but those aspects of his reputation aren't as well appreciated.
2. Sid Meier. The only person more popular IN THE CIV COMMUNITY is Sid. The community wouldn't exist without Sid. Sid is a modest guy, but he does recognize that he has fans, even if he doesn't believe he deserves them, and fans must have something to point at and say "That's so cool!" once in a while. The Sid Meier unit will put any city into permanent "We Love TheDS day" just by being in the city; joining is not necessary, Sid can move from city to city and spread joy. He's great at getting newly conquered cities out of disorder, but there's only one of him.
3. A Cat. I shouldn't have to explain this either; we all know that cats secretly rule the world and own us. Even those of us who don't host a cat in our own homes are aware on a subconscious level that all we do, we do for cats. The Cat will be able to turn any tile into a desert (litterbox), and can be sent to other civs regardless of OB agreements, where the Cat will enslave that civ without need to conquer it militarily.
Or maybe it'll be me. I saw someone suspiciously resembling Sid snapping pictures of me yesterday.