The strongest HUMAN psyker is gamma or delta. Space marines, primarchs, and certain exceptional individuals (malcador, emperor) are higher. Alpha-Plus rogue psykers have more raw force than even eldrad...when did eldrad psychically obliterate a battle-titan all on his lonesome, as alpha-plus psykers are described as doing? I don't recall the quote but I think its in the apocalypse book or somewhere and says they can destroy a planet with as much effort as snapping their fingers. And again, I never read eldrad as being a massively potent psyker in terms of sheer force, but in subtlety and skill. He can read the strands of fate better than even, perhaps, the emperor could, but I think that in a straight up psychic duel of power to power he would lose to ahriman or an alpha plus. Ahriman, btw, strikes me as lower than an alpha plus in pure strength but stronger again in cunning and knowledge and experience. Eldrad wins on those, but not on power.
And I'm unaware of Njal's fluff so forgive me for exlcuding him. Mephiston released all his psychic potential through conquering the black rage and so is very powerful and mentally strong. Tigurius survived contacting the hive mind and retained his sanity. Ezekiel, in addition to pwning a warboss on his own, can read minds perfectly.