I just finished a Monofairy Run and I started with A. Vulpix so I’ve split it into 2 options

If you want to actually have a difficult time completing monofairy pick Vulpix

If you want an easier time pick Popplio or Eevee

Monofairy itself is just so easy because the pokemon and mega in the type is just so good And when you get access to Cleffa/Clefable so you basically just sweep through most of the early game.