I GOT THIS
Haruka: Cotton candy is an obvious answer, but I feel like he'd also like birthday cake or a simple vanilla
Yuno: Strawberry or black cherry
Fuuta: Salted caramel
Muu: French vanilla or some sort of sherbert
Shidou: Butter pecan
Mahiru: I feel like she would like sorbet more than ice cream
Kazui: Coffee
Amane: Mint chocolate chip (she probably wouldnt eat ice cream BUT)
Mikoto: Cookies and cream
Kotoko: also wouldnt have ice cream but rocky road
Es: Pistachio or plain chocolate