Candidate for a merge project.

Lilina. She's already at +1, so you might as well finish.

Camilla and Marth have the highest BST of that bunch (158) so they would be the highest scoring in Arena with the merges. Since Camilla has really balanced stats, she would benefit the most from the blanket stat boost that merging gives you, but Camilla's Axe only benefits her when on a team with other fliers or cavaliers.

Personally I like Marth more than Camilla and I think that Falchion provides good coverage against dragons, and provides good Drive support when upgraded. I think ultimately it comes down what you need to fill out a team, but Camilla, Marth, and Eirika are all good options.

If you plan on merging Sothe, you miss out on good L&D fodder, Lilina is good but she's so dependent on ATK and OHKO that merging her really doesn't benefit her much, and Kagero is in a similar boat depending on how you choose to build her.

That Eirika has good IVs and Eirika is amazing with her refined weapon.

I will tend to Lilina or Camilla.

I have a +1 5* Lilina and she is really funny to play, Nuking almost everything not blue is always fun, and her tome give her a little utility with res chill.

An optimal Camilla is kinda expensive (DC primary) but she is really good, you can use her weapon for a mixed role build (utility/support/defensive or offensive) or stick with a slaying axe. She is very versatile.
I have a friend with a +10 full build camilla with DC and her axe with her refined option and honestly, she is reeeally good. Her mixeds defensives stats and spd make her hard to kill and with her weapon, she buff other ally nicely. If you stick to slaying axe/DC build, with iote shield she can counter effectively and kill rein and B!Lyn

Camilla will be more versatile but more expensive to build than Lilina, Lilina need just DB3 and WoM/SB3 etc... but she is a niche unit.

A budget Camilla is possible (switch DC with fury or the likes) but she will not shine as much as her DC build