The set of numbers that is always rational is integers. Rational numbers are defined as numbers that can be expressed as the quotient of two integers (where the denominator is not zero). Since integers can be expressed in the form of a fraction (for example, an integer \( n \) can be written as \( \frac{n}{1} \)), they are always rational.
Negative numbers and positive numbers can be rational or irrational depending on their specific values. Decimals can also be either rational (like 0.5 or 0.75) or irrational (like 0.333... or the decimal representation of π).
Therefore, the correct answer is integers.