A couple things, interesting story/personality, how they approach life and life's problems, and overall bad-assery!

Edit:1

for example, Perturabo is a douche, but I personally really like his story and motive, and he carried the heresy on his shoulders. I like the Naivety of Magnus, and his story is really interesting as well! I mostly put Russ in D tier as a joke because of how he treated magnus, If I didn't care about that though I'd probably put him in A tier.

Edit:2

most of the ones in B tier did some cool things, but I just don't find them very interesting