My #1 feat - The strongest mortal managing to fight and make a god bleed. The god then proceeded to kill the mortal almost immediately afterwards. But still it's regarded as the greatest and most terrifying feat.
Some honorary mentioning feats:
- Obtaining magic in a world without magic - During the dark ages of magic. The source of all magic, the arcanum, was severed from all the realms and resulting in the complete erasure of magic from every realm. One woman managed to bypass this severed connection and forged her own direct connection to the arcanum. She became the sole mage in the entire universe, a feat no one else ever achieved.
- Surviving the apocalypse - A dark god used his great power to instantly create a new sentient race in his image on the target world. But his power that flooded the world to create the race also had the effect of causing a cataclysmic event that literally ripped the Pangea apart into 3 separate continents and rendered the world nearly uninhabitable. Nearly all existing life was wiped out, and the environment became so... unlivable that most life that did survive the initial destruction either starved or died from environmental hazards. The race that was created was quite literally born into an apocalypse with no knowledge on the world, no knowledge on their own race, no knowledge of pretty much anything. They were created as adults, but with the knowledges and experiences of an infant. And they had to figure out how to survive an apocalypse at birth. Spoiler alert, most died. But their race as a whole survived long enough for the apocalypse to end and the world return to life. This is a huge feat of survival.
- Construction of the Quiron Bridge - The Universe is connected by threads/rivers/streams of a deep layer of subspace called "Quiron Space". Imagine a complex web of space rivers existing in a deep layer of subspace. Civilizations can ride these rivers to travel across the Universe extremely quickly. Travel between galaxies using Hyperspace can take decades or centuries, but can take minutes by riding on a Quiron Stream. But a battle between gods caused many of the streams to break, The gap between the broken ends of a stream could take centuries to traverse using Hyperspace travel methods. Which essentially rendered many galaxies inaccessible. This is where the Quiron Bridge comes into play. It's a planet sized megastructure built on both ends of the broken stream, and it creates an artificial bridge reconnecting the broken ends. Building a planet sized megastructure is a massive feat by itself, but this also required building the structure on the other end too, which as I mentioned earlier. Can take centuries to reach using traditional hyperspace methods. Once the stations were built and the bridge activated, then travel between the 2 stations would take seconds, but when building the stations they didn't have that bridge yet. In terms of architectural feats, this is the greatest feat by far.