The Campaign Builder's Guild

I go with history also. If you know where a culture has been, you know where it is heading.

I had to vote for the Multiple from Above option. Many of those things can, and do, fit together to create what a culture is.

Though, some things do it more than others. Race is somewhat low; a race of intelligent squids might have culture similar to any we have on Earth.

Politics is also low for me. Current politics always changes, and past politics becomes part of the 'History' category.

I think Alignment (well, i'll just call it 'Behavioral Traits' for now), History, and Notable People are the most important things to define, at least when taking part in an RPG. They affect how a culture acts, how they view outsiders, and the kinds of people the adventurers will encounter when within that culture.

I have to go with history on this as well.  History tends to have a lot of impact on what makes people tick.  Certain things that happened hundreds of years ago directly affect the way people do things in the present.

To further expand on my vote of history, while trying to detail   various nations for Kishar, I ultimately decided the project would become easier if I had a more detailed version of history than the summary I currently had (not that I think my history, myths, or timeline were sparse, but there's always more that can be added).  This desire spawned the Esras's Chronicles project, which is still on-going.  By writting history from the begining, I hope it will help me when I go to write more about the nations and races living in them.  For example, I started to examine how and why people of various races wound up in the places they did.

I cannot even imagine having done any of Kishar without the history I had already written before this.

Good luck.

EDIT: Question was answered.

Anyone else?

Yeah, I have to say the most important thing for a nation is something the PC's can bond with and use to enrich their experience and connect with the NPC's. History provides one of the quicker ways for this bonding as does politics and culture.

I have to say that language is most important, if subtle factor in building a culture.  I would agree with many of the posters on the answer of history, except for one thing:  If everyone was afflicted with amnesia and lost all historical context, they would still tend to create a culture much like the one they had before the plague of forgetfulness, because their thoughts would be channelled into patterns of behavior by their shared language.  History shapes language, true, but also vice versa.

QuoteIn terms of cultures, what is the most important element of a campaign setting?

History and language are nuances that may or may not interest or impact  the players, but politics are the here and now that can literally hold life and death.  It doesn't really matter the long list of past kings and queens you created to flesh out Kingdom Xia when the party finds their backs against the wall in the revolution.  What matters is the politics of the moment, not whose ancestor decreed Casual Fridays verbotten.

Language, history, religion, etcetera, are only important insofar as they impact the political environment of the current player environment.  They are all the chorus to Politics' leading role.