When you say objectively, do you mean closer to the platonic ideal of ice cream? Closer to THE ice cream, by which all other ice cream is known?

In that case consider: what of what makes ice cream ice cream is present in vanilla and chocolate versions? Moose tracks is a great ice cream, and it shares it's base with vanilla. But rocky road is also stellar, and it has a chocolate base. But in general, more ice cream types share the base with vanilla than chocolate. That would indicate vanilla is closer to the shared idea of all ice cream.

Further more, chocolate is a separate ideal of itself. No one desires the ideal vanilla extract, it is nothing on its own. So the ideal ice cream, totally disconnected from the material world and other ideals, would be closer to the blank slate vanilla than the specialized chocolate. Vanilla the word is even short hand for unaltered.

So in conclusion, from a deep examination of the platonic forms and ideals of ice cream, vanilla is objectively superior.

That being said, in my opinion chocolate is objectively superior in every way.