My immediate thought is Milady because she hides a huge-ass Gatlin gun under her skirt, but if we’re assuming that everyone is using their awakened Persona it requires a bit more thought I suppose.

I’m leaving out the protag because UHM. FUCKIN. SATAN.

Hecate is a strong contender with Concentrate and her strong af fire magic. Kamu Susano-o is the strongest physical attacker, but he has no means to heal himself, whereas Hecate at least gets a couple healing skills. Mercurius doesn’t hit as hard as the others and is a bit squishy, so even his healing wouldn’t keep him alive long enough imho. Astarte’s got a great advantage with both the -karn skills, but no Dia skills either and -karn skills use up a shit ton of SP. Anat is no longer rideable which sucks but she’s got some great healing and decently strong magic. Seiten Taisei is in the same boat as Kamu Susano-o, except kind of worse since he’s not as speedy. 

All that said, I feel as though Hecate and Anat are the strongest contenders. I’m going to say that Hecate would win, in my opinion. So, Ann.