To tell the truth, pre-H Guilliman should be one of the weaker Primarchs, he is stronger than Lorgar but he is probably weaker than the others, but it should be noted that when he gets angry he fights very well, he held off Angron for a moment before become a Daemon prince. Post-H is curious, all Primarchs are having a PU, the traitors became Daemon princes and know how to use their powers better, Corvus became a monster, it is not 100% confirmed but the Space Wolves have visions of a giant wolf in the Warp who is highly possible Leman Russ who has become a monster like Corvus, Lion El'Jonson returned with a hax shield and a new special skills. Guilliman is no exception, in a way as he obtained the Emperor's sword which is quite powerful against Chaos but the Emperor can possess him for a few seconds. I would say Post-H would still be the weakest but he has a weapon that allows him to fend off his traitorous brothers to some extent but the seconds he is possessed by the Emperor...