A town has three streets parallel to each other named X, Y and Z. If street X is not the busiest, and street Y is busier than street Z, which street is the busiest?

Let's analyze the given statements:
1. "street Y is busier than street Z" $\to Y>Z$.
2. "street X is not the busiest". Since there are only three streets (X, Y, Z) and X cannot occupy the highest position, the busiest street must be either Y or Z.
3. Since we already know $Y>Z$, Z cannot be the busiest. Thus, Y must be the busiest street.