[Faceman]

Jan: only thief in the game who can level up (ignoring Yoshimo for obvious reasons). IMHO must-keep if you have no thief as PC and continue to ToB, not because a thief is that necessary in the BG series but because I think every party should have one.

Imoen: Must-keep if you have a good aligned party and plan to roleplay on it. A good aligned character wouldn't ditch his sister in such a crucial time. Also a top mage (can wear elven chain and still spellcast)if you have no other (which would be Nalia if you have a good aligned party)

Aerie: Nice to romance if you are a paladin or the like. Yet multi-class chars are not so useful IMHO. Better to take Anomen for a Cleric and Nalia/Imoen (preferably Imoen) for a mage.

This is if you have a really good aligned party (Paladins, Rangers, etc.)
If you play a mix (good and evil chars)

Imoen: depends on how much sidequests you have done before getting her as she may be weaker in XP than your other party members. But she is still your sister.

Jan: Depends on how hilarious or annoying you find him. Still he is the only available thief.

Aerie: She has many party interactions (with Minsc, Haer'Daelis or YOU if you romance her). Keep her as a cleric if you can't stand Anomen.

If you are EVIL

drop Aerie because she is weak, pathetic and always whining