Was just about to say this. Being buried alive in a coffin, basically the plot of the movie Buried would be the worst thing that could happen to anyone.
Nothing could be worse than that, i would gladly take a beheading over that because thats instant your here one second then your gonna the next, burning alive would be more pain then you could imagine but you'll be gone quick.
But to be buried alive, locked up in a little coffin in the ground, not being able to move or see, no way to get out, knowing that no matter how much you shout nobody will ever find you, knowing you are certainly gonna die there alone from suffocating is terrifying to even think about.
Drowning aswell would be pretty frightening.