From what I understand there is a lot of overlap between Rig Veda Samhita and the other Vedas. Atharva Veda is considered a late addition to the Vedic corpus. At least linguistically, Rig Veda Samhita is the oldest Hindu scripture (although other works occasionally have older material).
Note:— This answer compares the greatness and result given by each veda as per prashna upnishsda and refrence from bhagawata-geeta rest all vedas are great.
Then Satyakama Saibhya asked him, "Godman, he who among men mentally fixes his mind upon Aum until the end of his life, which world he wins by that?
To him he said, "Satyakama, this very transcendental and immanent Brahman, indeed, is that Aum only. Therefore, the knowledgeable one arrives at one or the other through this means only."
If he meditates upon the one letter (A), having attained equanimity quickly by that alone, he attains the physical world. The Rics lead him to the world of humans, and there through austerities, the practice of celibacy, endowed with faith, he experiences greatness.
Now, if (he meditates) upon the two letters (AU) the attains the subtle world. Led by the Yajus through the mid-region, he is lifted to the world of the moon. Having experienced supernatural existence, he returns again.
Again, he who meditates upon the Supreme Self with the three letters of Aum, he becomes endowed with the light of the sun. Just as a snake is freed from its slough, in the same manner he becomes free from sin. Led by the Samans, he is lifted to the world of Brahman. In the congregation of living entities, he sees the highest of the high and the Person hidden in the persons.
The three letters are within the confines of death. They should be yoked and applied together. By applying (the unified Aum) in kriya yoga, the awakened one does not waver in the external, internal, intermediate and self-absorbed states.
The knowledgeable one knows this world by the Riks, the mid-region by the Yajus, and That (Brahman world) by the Samans. However, with the help of Aum, the learned one attains that which is the highest, calm, free from old age and death, harmonious, without fear and supreme.
It can be seen that om yet have place above all vedas.
वेदानां सामवेदोऽस्मि देवानामस्मि वासव: | इन्द्रियाणां मनश्चास्मि भूतानामस्मि चेतना || 22||
vedānām — amongst the Vedas ; sāma-vedaḥ — the Sāma Veda ; asmi— I am ; devānām — of all the celestial gods ; asmi— I am ; vāsavaḥ̣— Indra ; indriyāṇām— of amongst the senses ; manaḥ — the mind ; ca— and ; asmi — I am ; bhūtānām — amongst the living beings ; asmi — I am ; chetanā— consciousness Translation
BG 10.22 : I am the Sāma Veda amongst the Vedas, and Indra amongst the celestial gods. Amongst the senses I am the mind; amongst the living beings I am consciousness.