wizard or rouge
The wizard and the rogue have probably the most options. The wizard is everything the people said before, but rogues, they can create specialized poisons with a high enough skill check (10d6 con primary, with 10d6 con secondary, 1d6 weeks of paralysis, and harm spell in it... oh yeah, and DC 30 (using the rules from the quissential rogue or traps and treachery, I can't remember)). They can also use wands and scrolls with use magic device, making them versitle. The dividing the party with walls would work for a rogue. With improved invisibility, sneak attack can get annoying, and he could also be the leader of a guild, or a trap master. how about a trap with blades covered in the poison described above (ouch!).
The worst would be a team of nemesis all working for the same organization. How about an cleric/vampire/master of shrouds, a half-orc with sunder and other stuff, a wizard, AND a rogue.