Best:
In terms of design: I really like the Assassin's. I don't really know why, it just looks cool.
In terms of usefulness: Conqueror, Walhart's personal class. It's like the Great Knight, but Conquest negates the Bonus Damage done to Riders/Armored Units. (Of course, there's also DLC Ephraim)
Worst:
In terms of design: Sorcerer, specifically the female version (which for some reason is not called Sorceress). You can probably tell why.
In terms of usefulness: Sniper. Gets Hit+20, sure, but by the time you cap everything, you don't really need it, and it becomes obsolete by the Bow/Sword-wielding classes (like the aforementioned Assassin)
EDIT: Spelling when tired.