Depends on the cookie, the ice cream and.... the moment. Certainly not a hands down for either. Both, in the right context, can provide equal joy and satisfaction.
Match with cookie, not using ice cream but using anything else:
Brownie Sundae, brownies warm, with fudge swirls inside, topped with vanilla ice cream, warm choc fudge, whipped cream, one cherry on top.
Match using only cookie:
Ben+Jerry's Phish Food ice cream.
It is not even close. it is absolutely a hands down for ice cream!