In my book an optimised party has the following characteristics.

1. There is nothing every party member is bad at (stealth, social, exploration, and the various main combat toles).
And Any given task can be performed by at least 2 (3 if the group is large) party member

2. Every party member is the best at something

3. No party member has an obvious weekness that is not protected by at least 2 other party members