Actually, under some of the newer rules implementations it doesn't matter if you roll a 20 or not. Monsters that are large and greater in size requires greater massive damage rules to kill instantly. Such a dragon of collossal size would require 90 hit points done in one hit.
A long sword, lets say +5, weilded by a moderate strong warrior, lets say a 16 strength, would at maximum would only do 32 points of damage. Even if you were using the baseline massive damage system or the the d20 Modern massive damage there is no way a character could kill the dragon with a single hit on a criticial hit.