Well in my opinion:
FE4: Arden/Midir
(They are the worst parents in the game, Midir does to often and as for Arden, terrible speed)
FE5: I didn't get to play this out, but I must say Shanam is in no way even CLOSE to Shan an, his is just terrible
FE6: I actually played this game (without cheats) and every character is good when promoted, all except Wendy, Sophia becomes much much better when she promotes, she used Apocalypse for me, not Reigh
FE7: Erk is not a good sage, Pent is SO much better, and Nino is better as well
FE8: I'd have to say Gerik and Marisa, enough said.
FE9: Mist is the worst troubadour ever
FE10: Mist is the worst troubadour ever
FE11: Okay I saw a lot of negatives about this one, but I recently played it and I agree with one thing, too many cavaliers, vyland is by far the worst, Wolf re classed as a general is pretty good.
FE12: let's see, I really enjoyed playing this game, but my lease favourite character was roshea and vyland again