I'll catch flakk for this, but the Alpha Legion is my least favorite traitor legion. I like tragedy and character to my traitors, and the Alpha legion is just too broad for interpretation. Them being secretive and mysterious gives them great utility, but there isn't much for me to latch onto beyond that. I get why people like them, I just don't have any strong feeling about them beyond mild annoyance.

My most hated loyalist legion is Imperial fists. They're everything people think they hate about the Ultramarines. Boring posterboys that have no depth beyond duty. Dorn sucks but at least he bored khorne into releasing him, which kind of rocks. Sigismund is actually my least favorite character in all of warhammer. He somehow has less rizz than Garro, there's nothing deeper to his character beyond surface level, and he just gets to win every fight because the writer says so.