I think it's gonna be May. May and Coulson are kinda like parents to Daisy and since this is kinda Daisy's coming of age story and they've really been hammering home the family angle, they gotta abide by the rules and kill off a parent.
Also, May's kinda wrapped up all her storylines. There's the least consequence from a potential story perspective and greatest gain if she gets killed off. If she dies, Daisy can take up her role as the ultimate badass and struggle to deal with increased responsibilities and leadership. And also destabilize the entire team. Fitz or Simmons dying would really only affect whichever one survived.