The set of numbers that is always rational is integers.
Rational numbers are those that can be expressed as a fraction of two integers (where the denominator is not zero). Since integers themselves can be written as a fraction (for example, any integer \( n \) can be expressed as \( \frac{n}{1} \)), all integers are rational numbers.
In contrast, positive numbers, negative numbers, and decimals can include non-rational numbers (like irrational numbers). For example, the decimal representation of \( \sqrt{2} \) is a non-terminating, non-repeating decimal, which is considered irrational.
Therefore, the answer is integers.