Azure

Being an actual dancer would be better for Marianne, as Isolation is limited in the units it is on, and is only seen in AR, where you are likely going to see things like Pulse skills and Special/Crafty Fighter anyway. The skill is also useless in Resonant Battles due to proccing after combat. Wings of Mercy strats could be fun, but even with 70% damage reduction, 17 base Def isn't great.

Both 2nd place picks are my preferred units, as while Gatekeeper seems nice with his super obstruct B-skill, Martha Falchion is can be really stacked. He has good synergy with Fae as she can both counter his blue weakness and provide +5 spectrum buff to everyone.

While Eirika just has nearly everything a melee cav could want. Null Guard, player phase Null Follow-Up, Canto and a Slaying effect. If the healing effect was done on hit as opposed to Special procs it would be better as then Galeforce would be better.

I feel the others are gonna seem better off the bat but people will figure out ways to make Marianne much better

Agreed that Marianne would probably have been better of as a regular dancer. Her special can actually trigger in resonant battles, but that requires the foe to survive… which isn't ideal. But it bypasses the penalty/restriction of bringing multiple dancers for modes like resonant battles and grand conquest. Could maybe also be annoying in pawns of Loki. 

As for healing on hit with "Galeforce": it sounds a bit like a double-edged sword to me. On one hand you have a better chance of surving the next(/current?) combat. But on the other hand it makes it harder to remain below the HP threshold for "Wings of Mercy", and thus make it harder to sweep the enemy team.