I have dug up some information relevant to the discussion, because what psyker discussion would be complete without some eldar power.
Prepare yourselves for text!
Someone earlier mentioned the lack of information on dark eldar psykers. There is a good reason for this, which is elaborated on in "Path of the Renegade."
"The manifold gifts of the Eldar extended to very considerable psychic prowess and their ancient civilization had been built as much with thoughts as hands. But after the Fall the use of psychic powers became a sure way to attract deamons ... It was a hard vice to resist ... but the eldar kindred of Commoragh and its satellites soon learned to shun their psychic gifts, and to destroy those who pursued them despite the consequences. Now every scrap of their mental training focused on internalizing their powers and hiding their presence from She Who Thirsts."
Path of the Renegade pg. 30-31
So the dark eldar do not openly practice as psykers, as it attracts unwanted demonic attention. It is also important to note that the dark kin do not use soul stones, like their craftworld cousins, and instead are regenerated by regrowing their body by the haemunculi using a piece of the old body. It is possible that soul stones offer some kind of protection from the warp, but this is not elaborated upon as far as I can tell...
Secondly, there was some confusion as to who are the stewards of the Black Library are. This subject is touched upon in "Path of the Outcast."
"The Black Library of Chaos - Deep in the weft and folds of the webway is a craftworld unlike any other. It was to here that the Laughing God, Cegorach, first traveled when he escaped the clutches of She Who Thirsts. The scholars who dwelt upon the craftworld were surprised to see the god appearing among them, but he stilled their excitement and related to them his tale, and that of what had happened to the other gods. The Laughing God finished his narrative and disappeared, instructing the scholars and their protectors not to forget what he had told them. Thus was the Black Library founded, and the first of the Harlequins created. From the Black Library the followers of Cegorach travel far and wide, searching for all knowledge of the powers of Chaos and the manner of the dark gods of the Othersea. Artefacts of Chaos are brought here, for study and destruction, and within thousands of grimoires and tomes and volumes and tracts has been gathered an unprecedented literature concerning the warp and its denizens."
Path of the Outcast pgs. 147,148
So there you have it. The Harlequins are the masters of the Black Library, which has been canonized as a craftworld. This makes sense since it exists within the webway, like Commoragh, and the Harlequins are known to be masters of travelling the webway and its myriad pathways. Since it appears to be a vast collection of Chaos knowledge and artefacts then naturally the eldar would not want any Chaos sorcerers gaining entry.
As to the galaxy's most powerful psykers? Well, I am sure at least a couple humans would be up there, but considering that the eldar society nurtures and fosters growth of such naturally talented psychic beings, instead of stamping out most of them, then I am sure they would rank pretty high on the list. Unfortunately, since there has not been much eldar material released aside from codecies, then the main one that we know of is Eldrad, of course.