If this is your first (or second) run-through, I'd suggest staying with as many party NPCs as possible.
First, several bring their own quests into the group, including Mazzy, Korgan, and Edwin.
Second, quite a few have special items and/or abilities you can't duplicate without access to an editing program. Edwin's got a family necklace which gives him more spells per level than any comparable mage in the game. Valygar's got some fine armor. Haer'dalis has two swords with some decent extra effects.
Third, they have some great dialog and interactions which really make them come alive. That last does it for me.
