If we're assuming little to no grinding:
FE6 Best: Alan, Lance, Dieck, Rutger, Miledy
FE6 Worst: Bors, Barth, Wendy, Sophia, Dorothy, Cath
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FE7 Best: Marcus, Raven, Pent
FE7 Worst: Wil, Lowen, Heath, Karla
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FE8 Best: Seth, Ephraim, Joshua, Gerik, Lute
FE8 Worst: Neimi, Amelia, Marisa, Ewan