Which Coca-Cola products are most likely to lose costumers to Coke Zero?
Essentially, what you are asking is which goods can be seen as "substitute goods" for Coke Zero. Substitute goods are those that can be used more or less interchangeably. They are goods that both satisfy the same want or need. Therefore, you need to ask what current Coca-Cola products satisfy the same need.
Presumably, the first need being satisfied by Coke Zero is the need for a low calorie or no calorie drink. Therefore, Coca-Cola products that would stand to lose consumers to Coke Zero would include any diet sodas in Coca-Cola's line up. One might also argue that Coca-Cola products such as Dasani would lose customers since bottled water is, of course, zero calorie.
To answer this question, then, you need to determine what need you think Coke Zero will fill and then identify other Coca-Cola products that currently fill that need.