Firstly, I wouldn't try to find one great trait and play from it. This game is about adapting to different situations (i.e. maps, leaders, rivals).
That said, most of the experts agree that Financial and Philosophical are the best traits for most games, as both allow you to supercharge your research and keep a leg up on the AI. This is especially true of Philosophical, as the human player is infinitely better than the AI at planning certain Great People (GPs) to lightbulb certain techs (e.g. saving up 2-3 Great Scientists to lightbulb Education, then nab Liberalism for a free tech.)
Personally, I like big cities and I'm mostly a peacemonger, so my favorite trait is Charismatic. This would probably be so even without the military bonus, but that just makes it much better because now it's very flexible.
Protective is objectively the weakest trait in my opinion (not counting multiplayer.) Most people would also place Expansive and Imperialistic at the bottom of the list, and I mostly concur. However, I have been warming up to the possibilities of Imperialistic lately. It lends itself well to a "get 3 cities, then go to war" strategy. Even if the capital is food-poor, you can emphasize production and still have a good Settler pump. Then, once you start your first war, you'll have a military academy in one of your early cities rather quickly. So it has a pretty narrow early-game appeal, but I don't think Imperialistic is all that bad.
