This one is the best mainly because in the big pic you did the next: Shading and highlights are strong, cracks are big and strong, details are big, therefore they can be seen in the icon. The only problem is that the background dulls the image, if you really want to create a fiery effect totally remove the background, start with a glowing dark red and then go adding other colors such as amber, add small sparks and a little bit of yellow for highlights to create that molten core effect.