[Sever]

I am assuming that you are creating your party from scratch and not using the in-game NPCs.

1. Undead Killer: IMO there isn't enough undead in the game to warrant this as your PC. Having said that, immunity to hold and level drain will serve you well in many situations so if your happy with this, so be it.
2. Assassin: Very handy. You can backstab with any thief weapon but not weapons usable with UAI. Just don't use him/her as a tank.
3. Bard: Can be very handy but is easily rendered obsolete by a fighter/mage/thief. I always have a blade (Haer'dalis) in my party whether good or evil.
4. Cleric: Either pure-classed, kit or multi/dual with the class of your choice, a cleric will serve you well in countless situations. They even make effective tanks at higher levels. Recommended.
5. Sorceror: If you're experienced with magic use, this may throw the party balance of power away from your PC. (i like my PC to have at least 50% of kills and experience in any party). If you don't usually use magic, a non-specialist mage may be a better class to start with. Either way, you're gonna need a primary magic user.
6. Spare slot for quest NPCs.