system
Given the following post, respond as if you're commenting on Reddit:
human
[WH40K] who's the most powerful being that's not directly affiliated with chaos or the imperium?
gpt
Definitely the C'Tan. They eat suns. 

A Tyranid Hive Fleet could arguably be classified as an organism unto itself, but that's kind of a stretch. 

The most powerful non-human humanoid is likely Eldar Farseer Eldrad Ulthran, of the craftworld Ulthwe. All of his psychic abilities are top-notch, but his ability to predict and shape the future means that he has probably had the largest (albeit indirect) impact on the current state of Warhammer 40k of any being that still counted as unambiguously alive fairly recently. His achievements include the Second War of Armageddon, and the Tau.