Dragons feature in many different universes, with varying degrees of power, size, and strength, and where there be dragons, there be dragon scales.
Smaug (Hobbit book)
Smaug's body is covered in scales able to easily stop arrows, which probably impacted with approximately 52 J. He can also probably survive his own fire.
Temperature resistance: 1500K+
Blunt force resistance: Very Tough
Smaug (Hobbit films)
Smaug's scales are known to be incredibly durable, able to withstand glancing hits from a Wind Lance firing Black Arrows. If a Black Arrow weighs 8 lb, has a velocity of 14 meters per second,* and impacts over 1 meter, then it releases a maximum of 784 joules.
Smaug took several Dwarven grenade hits easily. Each blast probably has a yield equivalent to about a pound of TNT. He can also survive flying through his own fire, which is about 1400K+.
He has also survived being immersed in molten gold, which has a melting point of 1,064 degrees Celcius (1337K). Based on the behavior of the gold in the movie, it is probably at least 1400K. The pressure from the gold would also be immense, enough that incredible durability would be required to survive it.
*The bolt would probably travel faster in real life, so this estimate is a minimum. A real ballista bolt might travel as fast as an arrow, not as fast as a Nerf dart.
Temperature resistance: 1400K+
Blunt force resistance: Very Tough
Glaurung (the Silmarillion)
When Glaurung was young and not fully grown his scales were still vulnerable to arrows. Even at his full strength, his scales could not always stop the blows of Dwarven axes. His belly is basically unarmored. However, he can presumably survive his own fire, which is at least 900K, and probably much more.
Glaurung's belly armor is almost nonexistent, and can be penetrated with a sword.
Temperature resistance: 900K+
Blunt force resistance: Tough
Toothless (How To Train Your Dragon films)
Toothless can survive being bathed in burning methane, which has a minimum temperature of 1950C (2223K), though it is probable that not all the flames were methane. He also took a near miss or indirect hit from a Razorwhip's fire breath, which is bright blue and so has a temperature of around 7000K. He took a fire blast from a Whispering Death (~1300K), though he was slightly wounded, and three lava shots from Small Shadows (probably over 1770K) with no problem.
Toothless tanked getting hit by a Whispering Death's tail, which can slice through rock.
Temperature resistance: 1770K+
Blunt force resistance: Tough+
Red Death (How To Train Your Dragon film)
This dragon tanked huge boulders both falling and catapulted. It apparently lives in lava as well. Based on the glow of the lava in the movie, it's probably about 1270K, or more.
The Red Death can take plasma blasts from Toothless, about 3400K, though its scales were slightly damaged with each hit.
Temperature resistance: Between 1270K and 3400K
Blunt force resistance: Very Tough
Drogon (Game of Thrones)
Drogon can be wounded by a ballista ("scorpion") bolt. Therefore, he could probably be wounded or killed with a Black Arrow to the heart traveling at a realistic speed.
Drogon can also fly though his own fire, which can melt iron within a few seconds.
Temperature resistance: 1808K+
Blunt force resistance: Very Tough
Darkstalker (Wings of Fire)
Darkstalker has used his animus magic to make his scales immune to all damage, In the case that he is somehow harmed, his scales can regenerate quickly. He is show to be able to regenerate an eyeball in seconds, and to be completely immune to any physical damage other than his eyes.
Temperature resistance: Presumable infinite unless a more powerful magical being negates his protection spells—Very Strong
Blunt force resistance: Presumable infinite unless a more powerful magical being negates his protection spells—Very Tough
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