The short answer: An ordinary person. John Carpenter always wanted Michael to be an average-looking young man. He wanted to emphasise the juxtaposition between the truly heinous atrocities Michael commits with a completely normal human appearance, forcing the audience to digest that Michael is just a man. Michael could be anyone.
The long answer: While Michael has sustained many injuries through the Halloween series and likely has many scars, hiding scars was never the reason for his mask. Unlike Jason of Friday the 13th or Freddy Kruger from A Nightmare on Elm Street, Michael was never meant to look like anything other than an average person. While it's true we don't often get a glance under the mask (and the glances we do get are minimal at best), it's easy to see Michael is a tall, white male. In Halloween (2018), where he spent most of the first act without a mask, we get small glimpses here and there. He has short, grey hair and matching stubble. We do also see the scars on and around his left eye that were leftover from when Laurie stabbed him in 1978's Halloween.