Which of the following numbers is the smallest?

The smallest number among the given choices is -1, which is a negative number. In the context of comparing integers or real numbers, any negative value is less than zero and all positive values. Zero is the neutral point between positive and negative numbers; thus, it is larger than -1. Additionally, both 1 and 0.5 are positive numbers and are greater than zero and negative numbers. Therefore, -1 is the least among the options provided, making it the correct choice.