The ML selector is insane. I'd prioritize my thinking in this order:
1) Dark Corvus. He wins every Arena and GvG match on offense. Hard to build and incredibly niche in PvE (perhaps the hard tower or the world boss?), but he's the strongest offense PvP unit in the game. He's the closest thing in the game to an automatic PvP win.
2) Ruele. Amazing healer for all PvP. Makes every offense and defense team better.
3a) Judge Kise. If you like PvP cleave, she's the best in the game. Fantastic unit on offense.
3b) A-Vildred. The PvP cleaver that scares teams on defense and shreds on offense, plus can be used in b11/a11.
5) F-Cecilia. She's a wunderknight that can be built several ways in PvP (a key to a strong unit on defense). She beefs up your team and is plenty viable in a11, tower, etc.
6) MA Ken. He's a great PvE farmer but in PvP his role as counter-unit isn't scary enough on defense to rate him higher and there are better units on offense.
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Choosing any of my top 6 is mostly a matter of priorities and filling in gaps that you don't already have characters for. If you already have a strong cleave unit, then Kise and Vildred become less valuable. If your defense is what's holding you back most, maybe Corvus isn't the wisest choice because he excels in only one area and it's not that.
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Who knows if they'll ever nerf ML units like this again, so thinking long-term is important here: who will be most valuable no matter the meta? Arbiter Vildred and Fallen Cecilia get extra points on my list because they excel in multiple areas where they'll never be relegated to your bench. Dark Corvus offense will need a nerf to go away, but with Real Time Arena on the horizon, he'll be hard countered. Ruele is the safest choice and it's hard to imagine her PvP niche being infringed on too much, but already Achates + Chloe are similar enough to her and ML Ravi will have a revive as part of her DPS kit; also I think she's the least fun choice, no matter her undeniable power.