I voted Alpha Legion mainly because the book Legion was terrible and made me lose a lot of respect for them.
Secrecy for the sake of secrecy... 'master plot' that are so stupidly simple people don't figure it out simply because the Legion has plot immunity and/or relying entirely on
Contrived Stupidity. Oh, and the whole 'we have 2 primarch thing' is dumb. Especially since all they seem to do with it is have Omegon stand with normal marines, but because he's standing a bit behind, people don't realise he's 3 feet taller than them /facepalm. Oh, and right, we're super secret organisation that thrives on mystery... so when somebody join us, we fething BRAND THEM with an hydra... That has to be the most subtle thing ever! /double facepalm.
Maybe there's better fiction that redeem them out there, but based on Legion, I just can't take them seriously anymore.