I agree with Dojo boy that Industriousness is the single key trait. But the question as to what pairs best with it is still open: I am not convinced that religious (which means you would ONLY play Egypt, a restriction I do not like) is necessairly the only choice. I've perferred Military (hence choosing China, bonus, later GA generating UU), but can see where Scientific could be a possibility (see WoundedKinghts current thread on the no-temple no-cathedral approach: science with Indy vs. religoius with Indy). I've never tried commercial, and expansionist is one that doesn't appeal to me due to the short time frame (early on) that is it's niche. (No doubt Expansionist lovers will disagree!)
One has, I think, to admit that it is a totally COOL aspect of the game that by choosing different civ traits or combinations of them, one takes VERY different appraoches to playing the game. Ths for me makes Civ Three the hands down winner over Two. Still, I wish they'd done a better job testing the game initially and therefore avoiding the little maddening defects of the game, I will not list them. Although Civ 3 is pretty good, I hope they make it GREAT with the new editor and PTW.