system
Given the following post, respond as if you're commenting on Reddit:
human
[WH40K] If one of the Traitor Primarchs had remained loyal, which one would have been the biggest help to the Loyalist Forces?

Discounting Horus, because that's too easy.  And discounting Alpharius (because we all know he's secretly loyal).
gpt
Magnus

The only Reason Magnus turned traitor is because Horus Tricked Leman to kill him, forcing him to side with Tzeentch to save his legion.

If Magnus was loya (aka: never tried to use his Warp power to warn the Emperor and just send a messenger ship). Leman and Magnus would be free.

Meaning they could come and help Terra far faster then the Ultramarine who were busy fighting the Alpha Legion. Alternatively, they both would help the Ultramarine defeat the Alpha Legion faster, allowing them to reach Terra far faster.