Doomsday is a weapon that was created for a one and only one reason, to kill. He quickly rose to popularity because on his debut, he did the impossible and killed Superman. Even though Superman came back to life after a few issues but the impact stayed in the memories of comicbook fans. Superman was the superhero that faced difficult circumstances but never lost until he met Doomsday who was the perfect killing machine that was created by the Kryptonians themselves, the Els and Zods to be specific. Doomsday was created as a one time villain who would kill Superman and be killed by him but his rising popularity led to his revival in the comics.
Doomsday returned after he was killed by Superman but with his return he gained his greatest ability, adaptability. No matter how many times Doomsday get killed he will always regenerate and get resurrected with an immunity to the thing that killed him previously. This created the perfect monster, a monster that never dies, a monster that will always return no matter how many times he is defeated.
Doomsday is somewhat similar to The Incredible Hulk but much more powerful and with no mind or heart as he is made of pure indestructible mass. He only exists to kill, he doesn't speak, doesn't think, doesn't feel, he just kills whatever he sees. The Hulk gets stronger the angrier he got but Doomsday doesn't not get angry or sad or happy, he just attacks whatever is in front of him. This makes him much more dangerous and stronger than the Hulk. He was already the most destructible weapon ever created even before Superman killed him and he returned stronger than ever and he kept returning from then on, new and immune to what killed him the last time. This makes Doomsday the most destructible monster ever created because no matter how many times he is killed, he always returns and there will come a point when he will be immune to everything and be truly unkillable.