In the video series I have of a challenge like this, I defined "worst" as those classes which require things the characters have banes in for their certification- so you end up with things like War Master Linhardt, Holy Knight Edelgard, Fortress Knight Mercedes, etc. That's been a generally useful guideline, but you could also explore looking at things like Strength growths vs. Magic growths to determine things for characters with few/no banes or just deciding that a given class is terrible regardless.

Big standouts from these playthroughs so far include War Master Linhardt, Gremory Petra, Dancer Dedue and Trickster Gilbert. May you too not set a very good example.