Given that CSMs hate everyone and everything, what CSM legion is hated most by CSMs?

If anyone can think of a better way of phrasing that, do let me know haha.

Edit. I was wondering if the Nightlords had a place in the most hated in the current setting since they tend to avoid warp shenanigans, are few in number and are particularly backstabby.

Compared to the Emperors children though it could be small potatoes.