* λογικός (logikós): This is probably the closest single word. It comes from *λόγος* (logos), meaning "word," "reason," "speech," "account," or "principle." So, λογικός refers to something that is logical, reasonable, or based on reason. This is often the best choice.
* ρητός (rētos): This word means "expressible," "articulate," or "intelligible." It's related to the mathematical concept of a rational number (a number expressible as a fraction), so it could be appropriate in that context.
* συνετός (synetós): This means "understanding," "intelligent," or "sensible." It emphasizes the ability to grasp rational thought.
The choice between these depends entirely on the intended meaning of "rational." If you're talking about logical reasoning, λογικός is best. If you're talking about a rational number, ρητός is more fitting. If you're referring to someone with good judgment, συνετός might be preferred.