Bhishma knew Krishna’s identity much earlier than anyone else.
In Sabha parva he elaborately explained why Krishna should be the one to take first worship.
Hrishikesha is the officiating priest, the preceptor, the bridegroom, ^ the snataka, the king and the friend. Therefore, Achyuta has been shown the homage. The worlds owe their origin to Krishna and in him are they dissolved. It is in Krishna that all the beings of this universe are established. He is passive nature, he is the active doer and he is eternal. He is supreme among all beings and it is for this reason that Achyuta is the eldest. Intelligence, mind, greatness, wind, energy, water, sky, earth and the four kinds of beings^ — are all established in Krishna. The sun, the moon, the stars, the planets, the directions and the intermediate directions — are all established in Krishna.
Ch 35 Sabha parva Bori critical Edition
Bhishma also knew the Nara Narayana origin of Krishna and Arjuna through Narada.
the same way, Narayana killed many others in that battle. Behold! These two immensely valorous ones have arrived as the maharatha warriors Vasudeva and Arjuna. It is said that they are the ancient gods Nara and Narayana. They are invincible in the world of men, and even against Indra, the gods and the asuras. It is said that Narayana is Krishna and Phalguna 5 " is Nara. Narayana and Nara are one being, but have been divided into two. They have earned eternal and indestructible worlds because of their deeds. When the time for war has arrived, they are repeatedly born there and here. This is the task that they have to perform. That is what Narada, knowledgeable about the Vedas, told the assembled circle of the Vrishnis.
Ch 48 Udyoga parva Bori critical Edition
Bhishma took permission from Krishna to leave the body.
“O illustrious one! O god of the gods! O one who is worshipped by the gods and the asuras! O Trivikrama! O wielder of the conch shell, chakra and the club! I bow down before you. O Krishna! O Vaikuntha! O Purushottama! Grant me permission. Protect the Pandaveyas. They are devoted to you.
Ch 153 Anusasana parva Bori critical Edition
After him I think Kunti and Yudhishtir got to know Krishna’s reality.
Kunti said to Krishna
O Krishna! I know about the influence of your truth and your noble birth, and the intelligence and valour that you display in establishing your friends. In our lineage, you are dharma, you are truth and you are the greatness of austerities. You are the saviour. You are the great brahman. Everything is established in you. Everything will be as you say. Truth is established in you.” Govinda took his leave of her and circumambulated her. The mighty-armed one left for Duryodhana’s residence.’
Ch 88 Udyoga parva Bori critical Edition
Kunti prayed to Krishna to revive stillborn Parikshit. She knew he is all powerful supreme god.
Yudhishtira knew Krishna’s identity but so did other Pandavas including Draupadi.
Arjuna’s praise of Krishna in Vana parva proves the point. Same with Draupadi who acknowledged his omnipresence through her prayer.
So Bhishma leads the list in this regard.