[Faceman]

every party should have at least
1 mage (single/dual) for identifying and aggressive spellcasting
1 cleric or druid (single/dual) for healing, group enchantment, turning undead
1 thief (single/multi/dual) for traps and locks
all other chars are free of choice
some tanks for protecting the above, melee and getting rid of easy foes without wasting spells.

the best NPC for clear aligned parties are

good:

tank:
Minsc (Strenght, Dual wielding) and/or Keldorn (Carsomyr, True Sight)
mage: Imoen and/or Nalia (Dual class enables them to use more items and wear elven chain)
cleric: Anomen (gets enhanced stats, can be used as tank as well)
thief: Imoen (for storyline) and/or Jan (levels up)
support: Haer'Daelis (bard song, dual-wielding, off. and def. spin) and/or Mazzy (very fast with short bow)

evil (there's only one for everything anyway):
tank: Korgan definitely
mage: Edwin (lots of meorization slots, but singleclass sadly)
cleric: Viconia (powerful spells, can wear human skin)
thief: Yoshimo (pure thief) or Jan (for some extra magic missiles)
support: Cernd (nice druid spells and extremely mean as a greater werewolf)

You get top results if you mix them and don't forget that the most essential char is your PC, as you can fit him to your needs