Best F2P Magical Tank?

Fae is great. Fjorm works.

Arvis and Julius can actually tank Reinhardt with distant def and the right build.

I was very successful with Berkut's Lance + DC Clair before she got a new weapon. She also has Blue Tomebreaker since Reinhardt tends to use Lancebreaker.

Florina, Mathilda or Nowi should also work. This is obviously assuming that you want a tank that can tank both Reinhardt and Lilina.

A green unit would be the best against Reinhardt, but you'll have problems finding one that can also take on Lilina, if you can even find one.

A well-built Julius might also work.

Felicia can do good work with her high Res and Spd. With the special refine she'll hit Reinhardt with a Glacies is he initiates, though watch out for QP/Moonbow.

I know you said merge project, but...

one word:

Julius
literally THE best mage tank in the entire game

Oliver. Triple distant def, or if you lack blarserpent, blarowl is pretty good.

For clarity, I'm not joking. He can achieve upwards of 50 res, even 55, without merges or buffs.