Use 1 for normal humans. Or A. Or Alpha. Or Aleph if you want to be venturesome. And build from there.
Because virtually every superhero novel I've read where they've had a classification (comics, on the whole, do not), they make it the highest rating and classify downwards. And every one of them hits the same problem as stellar magnitude and D&D Armor Class:
Then you get something better (or brighter). With numbers, zero starts to get silly, and negative numbers are worse. Alphabetic -- you have to drop the system entirely. It makes the characters in the work look silly, that they didn't build it for increases.
(Stellar magnitude does have the opposite issue as well -- having rated all the stars from 1 to 7, we also, after telescopes, had to deal with stars dimmer than 7 as well as brighter than 1.)
Oddly enough, the only rating I've seen that goes from 1 to 5 has the advantage that 5 is "powerful enough to be threatening to an entire dimension at least" so it wouldn't have to go higher. . . .