Well there was a great thread on this topic that got posted to a web site:
http://www.nomiata.com/iwd2/IdealParty
Basically the "formula" is to go with 2 "Tanks"; 1 "Leader"; 1 "Sneaker";
1 "Healer"; and 1 "Blaster". If you fill those roles with a starting party, you'll have all your bases covered.