Why is Odysseus said to be the cleverest of the Greek heroes?
Odysseus is known for his cunning due to his strategy, his trickery, and his, ahem, relationship to the truth. These all come together. You can see them in the ruse of the Trojan Horse that won the Trojan War. It was his plan to come up with this great gift to Troy, and also to hide the ships, so the Trojans would think the war was over. This isn't meeting on a field of honor, strength to strength. This is winning through trickery. You can see this throughout "The Odyssey" as well—look at how he tricks Polyphemus, how he changes his name, his story, his tack…
Odyessus is considered the cleverest of the Greeks for many reasons, not least of all because no matter how many times he messes up, he finds a way out of the mess.
It was Odyessus who came up with the idea of the Trojan horse, and perhaps even how to get the Trojans to take the horse into the city in the Aenad.
It is that same man who, while bungling with Cyclops, figures out that if he ties his men to the underside of the sheep, the blinded Cyclops will not realize they are escaping.
If you go through the Odyessey, you will find other instances of how Odyessus used clever techniques to get out of tough binds.