As said, as time passes by you'll have ALL the MP you'll ever need. I have all the Souls maxed out now, and all the skills I'm even remotely interested in.
But since you're not at that point - well, Herc/Shingen(/Mordred/Andro/Joan) comes first. It's just a matter of which Soul you use the most.Joan and Andromeda are vital for Rag raids and beyond. If you're not strong enough to MvP, please bring Andro/Joan. No Ascension is necessary for Andromeda, since if the boss hurts that much, you're better off bringing Joan.Not really a necessity, but for Tower, having Solomon offensive Relic weapon and 21 MP spent on her (6 points in Ability+ and her CD reset skill) is a game changer for lower floors.