What do you consider would be the worst party? (Or, if not the actual worst, the people you would least like to have in your party.)
I'd go with:
Cernd - A solid choice. Not much use except when shapechanged.
Haer'Dalis - I don't know how good or bad he is as such, but I wanted to take his ego and strangle him with it within minutes of speaking to him. Most irritating NPC in the game. I took great pleasure in telling him to bog off back to wherever he came from.
Yoshimo - It's true he has decent thief abilities, but I prefer Jan in that capacity. Also, he seems to run away from combat at the earliest opportunity regardless of what AI you assign him, so not much use as a fighter either. Last, but not least, what he does in Chapter 4 would also weaken your party overall.
Anomen - Another one whose ego is too large to fit in the same room as him. Not too bad in combat but not great either, and there are several much better fighter types (Keldorn, Korgan, Minsc) and a better cleric (Viconia).
The fifth and final choice is a bit trickier for me as all the NPCs that are left have some merit. At a pinch I'd probably say Aerie, because she has poor HPs and is more likely to snuff it than anyone else. She whines a heck of a lot too.
As for the protagonist, I'm not sure which class I would least like to play. Pure Druid perhaps. Not a lot going for it that I can see. Assassin doesn't do a lot for me either.