There's a lot about the nature of psykers that is unclear. What exactly are Dark Eldar Psykers like, where do the old ones fall into this and does the Hive Mind qualify in the semantics of the question? Does the nature of the Emperor as a psyker parallel the Chaos gods enough to disqualify them. I believe that the Emperor is more powerful than anyone of them and even rivals them combined.
Honestly, most of these debates can be rebutted by reading the various codexes.
For any player who owns both the CSM and SM Codexes, its interesting to see the differing perspectives. Bot Codexes tell the tale of the Horus Heresy, but tell it very differently.
The CSM codex paints Horus as being a rivel to the Emperor in every way, and with his gifts from the Chaos Gods he was the Emperor's better. They describe the Emperor as having made many mistakes and besting Horus only by chance of Horus hesitating and the Emperor taking advantage of the window of opportunity.
In the SM Codex, it tells the stryo differently. It makes it very clear that the Emperor was in every way superior to Horus (at his peak, and while possessed of all for Chaos Gods). There is a passage that describes the momment when the Emperor defeated Horus, I can't quote becouse my codex isn't handy atm, but it said something like - "The Emperor gathered all his might and obliterated Horus from the mortal plane, all for chaos gods recoiled in terror and fled their mortal pawn for fear of dying themselves".
That's a strong statement, and says a lot about both sides.
Obviously the codexes are meant to tell two sides of the same story, and while one may be more 'correct' than the other, I am disinclined to beleive either one.
However, what BOTH codexes make very clear, is that the Chaos Gods... collectively, feared not just the Emperor, but his clone sons and what they were capable of.
I would definately rank all for Chaos Gods as below the Emperor in psychic might, collectively. I personally don't think its far fetched to say that they may rank below even the primarchs, but more likely, they are probably equal on many levels, and due to the primarchs weaknesses, thay are able to control or manipulate them.
Malcador is sort of a dead issue sinse he's dead, but he was the only person in the Imperium that the Emperor placed no restrictions upon for use of psychic powers (which he did not want Magnus using - also a telling statement).
The Eldar are definately a strong psychic force (Slaanesh and what not) and could definately produce rivals to any Imperial psyker, but the Eldar fluff is full of inconsistancies.
Eldrad is definately a powerful psyker, and could probably take even the most accomplished of current Imperial and Chaos psykers. But he is also the Eldar's premier Psyker.
I would imagine that Tigurius and Mephiston are next, and then Arhiman.
Tigurius, contacting the Hive mind means a lot, and in terms of his profile, I find it hard to beleive Tigurius would loose in a duel against Ahriman.