Agamemnon king of men spoke thus: “Diomed,” said he, “son of Tydeus, man after my own heart, choose your comrade for yourself - take the best man of those that have offered, for many would now go with you. Do not through delicacy reject the better man, and take the worst out of respect for his lineage, because he is of more royal blood.”
~ The Iliad - Book X (Samuel Butler Translation. p67 / v239)
The Iliad Book X. Diomed shines again. He is the first to volunteer when they decide to send someone on a risky journey to spy on the Trojans. He and Ulysses raid a part of the Trojan camp and return.
In book 10 of The Iliad, Diomed (Diomedes) volunteers to spy on the Trojan camp to get more details about the enemy. Many others volunteer to go along with him. Agamemnon asks Diomed to choose one person and choose based on merit and not on lineage.