Well, HP can't be improved with equipment so I think high base HP is useful though I find Attack to be most helpful. For instance ignoring skills and abilities I think Star Lily has the best stats for a 5* pierce. As Ikki said Def needs to be high to be useful because it reduces damage by 1/4 your Def value, so it won't make much difference unless you can block a big chunk of the attack. Atk/Def is good for girls with life drain skills too, I've seen Alpinia take out the 12-5 boss by herself.

But stats don't matter as much as skill and ability (if they are the same number of stars) so I choose based on that first. I have a Lily Rubellum who has great attack for a pierce type but I swapped out for Mountain Lily because her skill and ability does more damage even though she has a lower base attack. And I'll be replacing Mountain Lily with Toad Lily soon because Toad Lily just has top tier abilities even though ML and Star Lily have way better stats and full equip slots. Then there's Edelweiss who has great everything, stats, ability and skill.