it totally depends on the circumstances:
if Renka was just stood up or dumped by that pretty boy princess, then I'd sat she deserves the ice cream most
if L has just fingered Kira as Light (lol) then I'd give him the ice cream as a reward
and if Yostuba was around I'd just tell her to steal the ice cream from whoever I just gave it to
otherwise Yostuba gets it every time, amirite?