Efficient-wise I think these are the best/worst units:
FE6 Best: Allen, Lance, Melady, Perceval, Larffin
FE6 Worst: Sophia, Cath, Wendy
FE7 Best: Marcus, Lowen, Sain, Kent, Pent, Priscilla, Raven, Ninils
FE7 Worst: Karla, Nino, Wallace, Wil, Rebecca, Rath, Renault
FE8 Best: Seth, Franz, Vanessa, Ephraim, Tethys
FE8 Worst: Ewan, Amelia, Marisa
Of course, units with higher movement tend to do better, so they're the better units in the game, bad units are just worthless, never worth investing into,detrimental to even use them.