If Maupay himself has the pick of Forest, Everton or Fulham, he'd be mad to go anywhere other than the Cottage - discounting different wage offers, obviously.
Everton - massive pressure on whatever striker they sign, to perform. Unrealistic, miserabilist fanbase. Unproven, one-dimensional manager.
Forest - have signed a truckload of forward players, plus he'd be in direct competition with the excellent Brennan Johnson, meaning he'd probably get less game time than staying here.
Fulham - reasonable levels of expectation. Nice place to live, if indeed he needs to move at all. Could form a fantastic partnership with Mitrovic. As a pair they'd be horrible to defend against - Mitrovic's strengths, complemented with Maupay's movement and tireless pressing.
Just based on that, I think Everton would be his choice. He wants to be a hero, not Mitrovic sidekick.