Why isn't it the Blood Angels that are the poster boys? If not for Sanguinius hurting Horus enough, the Emperor would have never been ablet to take hime down. Guilliman was a dumbass who got tricked by the Word Bearers and kept out of most of the fighting as they were held back. They even lost to the Alpha Legion and had to resort to orbital bombardment to finish them off.
Blood Angels tricked by Horus, and again it was Horus who told the Ultramarines to gather at Calth where they were attacked by the Word Bearers
Its only because they were so far away, and arrived long after the battle was already won by the other chapters, that they were the only ones left at full strength, and Guilliman seized command as the only one with the power to back it up, not because he was chosen to do it. Heck, three of the greatest primarchs, Dorn, Vulkan and Russ thought he was an idiot and didn't support his plans until they were forced to. Dorn having the Imperial Fists attacked on all sides by the imperial navy being the only reason he gave in.
Well first the Imperial Fists weren't attacked on all sides, it was one Imperial Fists vessel fired upon by another Imperial Navy ship- neither was said to be destroyed and nothing suggests that Guilliman ordered it.
Guilliman was backed in his plans by Corax and the Khan (who had also stood and defended Terra alongside Dorn). Gulliman might have incurred relatively few losses during the Heresy itself but between the ending of the Heresy and the break up of the Legions the Ultramarines were severly depleted (somewhere between 75-90% of the Legion was destroyed) holding the Imperium together.
Heck... Then later the Ultramarines got their asses handed to them by the Tyranids on their home planet and only just beat them back... yet, they're claimed to be the greatest? Yeah.. right...
An entire Hive Fleet attacking in one hammer blow! Other Chapters have lost their homeworld because of a massively smaller splinter fleet invading, yet the Ultramarines along with the Ultramar Guard and the Battlefleet of the Segmentum just managed to hold off millions and millions of tyranids- at a time when there were no anti-Tyranid doctrines and they were a virtually unknown aggresor I might add.