Best
FE6: Rutger. His Hard Mode bases are pretty great and Swordmaster is quite the powerful class in FE6 with their high Crit rate. Runner ups: Miledy, Deke, Percival
FE7: Hector. Strong, durable, has a very powerful Sacred Weapon and unlike Eliwood he can be pretty godly even in his own route where you are forced to use him. Runner ups: Raven, Lucius, Pent.
FE8: Seth. I mean cmon. Oifey, strong starting bases and very good growths that carry him for the WHOLE GAME. Bringing him to the final chapter is not a bad thing to do. Runner ups: Joshua, Cormag, Eprahim.
Worst
FE6: Sophia. Her Con, bases and the weight of her weapons speak for themselves. Runner up: Wendy
FE7: Karla. Terrible join time with some low bases. Runner up: Renault
FE8: Knoll. His stats are pretty bizarre and it doesn't help that Gleipnir is the worst Sacred Relic in the game. Runner ups: Amelia and Ewan.