What are the most important things to bring on a ride?
A mobile phone. These days, irrespective of where you're riding, what you're riding and over what terrain or distance, there's no excuse for being deliberately out of contact. We can't guarantee being in an area of reception, but if you haven't got a phone you'll never know. 

Additionally, a huge benefit of smart phones are the apps that can do more than just call for assistance. e.g. I have an app which lists all bike shops in London, with maps, reviews and opening hours. A couple of times I've been stuck in the middle of somewhere unfamiliar and in need (after the second puncture of the day without a second spare tube): fire up the app, where's the nearest open shop and then it can lead you there.