What is the easiest and quickest way to pump up a tyre?
If you're at home, use a floor pump. They are very easy to use and have a gauge so you can measure the pressure.



When you're on the road, the quickest and easiest way to get you back on the road is to use CO2 cannisters. They are not that cheap though and the CO2 will leak out of the tube faster than air (mostly N2), requiring topping up over the next few days.



More information is available on Wikipedia