All signs point to pre-Aurelian Lorgar being the weakest of all Primarchs, even Alpharius, the smallest of his brothers, is considered more of a tough guy. Angron, Guilliman, Ferrus Manus, even Lorgar himself consider Lorgar the weakest Primarch. This changes with Aurelian though, when he crushes Fulgrim (Granted, he had the initiative), and proves to be second only to Magnus the Red in psychic power.
MK Raiden? Worse than MGS Raiden lol.