[SoL Berzerker]

I'm pretty sure Malcador was only a human, and he was the strongest psyker short of The Emperor and Magnus. I'm pretty sure he was described as more powerful than Space Marine Librarians.

He was able to get punched by a Primarch, and then not only keep himself alive but walk purely through his psychic might. More than what many Astartes Librarians can boast.

EDIT: But to answer the question, so far Malcador is the limit, outside of The Emperor and Magnus. I'm not to sure how Lorgar compares to Malcador though.

 

[SoL Berzerker]

I'm not sure there are ever any comparisons between Malcador and Magnus' power levels.

The only reason I put Magnus above Malcador is because Magnus was intended to sit on the Golden Throne for a very long period of time... Like thousands of years. Malcador managed a few hours at most.