If this is solely just for Lucina and meaning no Cynthia is involved in the rating nor Kjelle, and because it's for min max purposes, I'd give....
Sumia!Lucina an S rank Sully!Lucina a D rank.
Reason being is because Sully forces Lucina into being a physical damage dealer without the use of Magical Swords or anything similar. Sully also loses out on Galeforce for Lucina.
With Sumia, you get everything + the ability to abuse Celcia's Gale.
The main reason why I don't do this pairing though is because I'd rather have an Inigo with Rightful King as much as people say he doesn't need it and prefer Cynthia to be a magical damage dealer personally. --- LoL NA: Lurkerkiller SC2 NA: Lurkerkiller.301
Sully!Lucina: B. For reasons explained. Awesome in no-grind, otherwise limiting.
Sumia!Lucina: S. Olivia's advantage over Sumia offensive wise is overrated. The biggest advantage Olivia gives to Lucina is the Assassin class. Most users don't seem to bother with Lethality/Astra, and vantage isn't that great on Lucina anyways.
Aether/Luna is all the offense Lucina needs, especially against capped enemies due to their significant defense. And as mentioned: Sumia gives Lucina far more versatility than Olivia. --- Official claimant of Mem Aleph for the theoretic cause of promoting Shin Megami Tensei IV of the 3DS.