Aside from reading the amazing guide there I think you should also consider your own personal preferance. For example with your character are you wanting to do pk?
Quests?
Is it about the look or the story of your character?
For me wlo is my own little world and sometimes I get pets based solely on what I like. I quite like the way kanako looks and she is so pretty when reborn. So that my input! Haha but yes read the guide for stats etc.

Also remember any pet can be really strong when fed from the IM and then scrolled with pet lethes.