My opinion is Bio-exorcist is the best. She has access to percent based damage, healing, revive (sort of), wither, curse, armor break, immunity, and a command grab. Strong on both attack and defense, the only card rated SS tier for both on the community tier list.
How does she benefit from diamond:
You don’t want all her damage to come from the SA1 or else you’re taking 50 seconds per enemy and high risk to time out.
Larger health multiplies her healing.
I certainly think that's a good choice, and I appreciate a well thought out analysis!
At this point, though, I'll be sticking with Primed. Her SA and general concept of pure damage (rather than the % based damage of Bio) would benefit the most from the stat boost of diamond status. Well, her and Wulfsbane. Bio will always be able to deplete at least 20% of the opponent's HP in ten seconds, high stats or no. WB and Primed could, but it's reliant on stats. Bio is definitely a great choice, and it wouldn't be a mistake to diamond her first, but she isn't my top priority.
Also, if I'm being honest, I don't quite think Bio is an SS tier defender. Unless, of course, she was Diamond, then she's never been much of a threat for me. Usually she doesn't even get the time to charge, and if she revives, she's probably already stuck in a combo and just took off a chunk of her teammates to help me out.
And to address the time out issue: Valid point! Ideally, though, I'm already using her on a team with BHD for perma bleed or some other damage dealer, so it wouldn't be just her doing the work. And, as I stated before, I won't be using her on defense, so I'm not worried about her not being able to hold as much of the enemy's HP.
Tl;dr: Bio's SA makes her good even without outstanding stats, but Primed/Wulfsbane is more reliant on them and therefore would benefit more from the Diamond stat bonus.
