I almost went for Effie (who required training up anyway), because I do want a super effective physical tank, and Effie would be great for that since she's a blue unit and the vast majority of enemies are physical red units.
However, I realised I can, for the time being, just give Nowi Triangle Adept, since that apparently trivialises even Falchion users on her. I think I have Selena so I'll use her.
Reinhardt has been a unit that I've had a mind to promote for a long time, but I wanted to build up physical units first. Then I played Echoes and realised I had a 4-Star Clair, and just had to have a 5-Star one. :V
But he's an absolute nuke against everything that isn't a green mage, so he's obviously gonna be really useful, plus he has the extra Mov. He's also pretty much ready to go out of the box, especially since he has Vantage without Inheritance. My Ryoma and Jeorge have Vantage 3 too so adding Reinhardt to the list will just make Enemy Phase a lot easier, plus it gives my Elise and Sakura a good potential niche in certain fights. I also just needed good cavalry units so Reinhardt seems as good a place to start as any.
Should note that I'm mainly building my units up for Tempest Trials and stuff like GHB's and some of the more difficult free Orbs missions. And since I've heard people having a lot of success with Reinhardt in that regard, he seems like a no-brainer.