Coke Zero is a newer diet option formulated to taste similar to original Coke, with the presence of a mild artificial sweetener flavor that is more recognizable to those who are sensitive to the taste. Diet Coke, on the other hand, was created decades before they had decent sweeteners. While it still has a sizable following of people who like it, Diet Coke tastes nothing like original Coke, and has a flavor more akin to the smell of hot plastic.

This is pretty consistent across all brands now, at least from those that I’ve tried so far. The new Zero versions are much, much closer to the original formulas.