How to treat others with respect. I know it's vague and it's a bit much to ask (because they're taught that anyway but refuse to take it on board), but I think it's the most important because I feel if there was more peace in the world, no fighting, we all worked together etc. We'd be far more advanced at this stage than we are right now. Everything is about working together and that's exactly what we should be doing.
That being said, it would be great if everyone learned how awesome Irn Bru is, but I guess I'm lucky living in such close proximity to Scotland