Um.....
What?
I would suffocate if there was no air, I would starve if there was no food and I would dry out without water, so I can't really choose anything. If either of those things vanished from the planet like you seem to be implying, everyone will die.
So, following that same logic, it will have to be food. It would kill everyone in the slowest amount of time. If I took away air, everyone will die in just a couple of minutes. Without water, you can usually survive for about 5 days. But if I took away food, it would take about a month for everyone to die. It's the best I can do for everyone here on Earth.
...But I may be a little sadistic.