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Given the following post, respond as if you're commenting on Reddit:
human
[Warhammer 40K] Who would the Ultramarines regard as acceptable substitutes, if they had no men to spare?

That is, if there were no Ultramarines in the vicinity of, I don't know, Guiliman's used hanky that they desperately wanted to reclaim from the forces of, let's say the Eldar but it can be anyone. Are there any other chapters the Ultramarines could call in and trust with this vital mission? Who would be their preferred pick?
gpt
I , CATO SICARIUS, MUST REMIND YOU THAT AS PER THE HOLY CODEX ASTAARTESS US ULTRAMARINES NEVER REQUIRE SUBSTITUTES, FOR WE ARE THE GREATEST OF THEM ALL!

COURAGE AND HONOR!

\*Ahem\*

Canonically, Guilliman had a huge amount of respect for several of the other Primarchs, including ones you wouldn't expect like Leman Russ and Khan.  But that was mostly his personal opinion. Since the Blood Angels and the Iron Hands (his other *openly* liked chapters) have kinda gone all non-Codex and possibly heretical, the obvious choice would be the Imperial Fists and successors. They get along just fine with the Ultramarines (~~and are also technically more Codex-Compliant than the mainline ultramarines are~~ **HERESY!**).