Once I read that the strongest thing in the Marvel Universe is Silver Surfer's Skin, followed by the Mjolnir and Adamantium. Is this right? And, if so, what makes them so special?
Proto-Adamantium: The adamantium-vibranium alloy used in Captain America's shield is the strongest human-made material on Earth. It's greatest strength is its nearly immutable nature and resistance to being damaged or changed. The process used to create adamantium requires the material be brought together, shaped and within a fixed amount of time before setting. The process used to create Captain America's shield was similar with the inclusion of vibranium. The process was lost and therefore unable to be repeated.
Enchanted uru is not made or normally found on Earth and is both more resistant to damage than normal uru metal but it is not indestructible. It's durable and can be enchanted to be even more damage-resistant but its primary benefit is the ability to bind amazing enchantments to it.
Thor's hammer vs. Hercules' golden Adamantine mace
To be fair to the Olympians, I will also include Adamantine (the material Hercules' mace is made from) to be another mystical metal capable of being enchanted and having its durability enhanced through mystical/scientific forging by Hephaestus, the smith of the Olympian gods.
Galactus created the Silver Surfer, so it's safe to say the Silver Surfer is by far the most resistant to damage, considering his job duties and the potential threats he faced, he needed to be. Since he and Galactus both possess matter transmutation and advanced scientific capacity they can create or alter matter to make the strongest materials in the Universe. Other cosmic beings can likely create similar near-ultimate materials.
In the current iterations of the Marvel Universe, the nature of the most powerful materials would be similar to the nature of the technological capabilities of the species, so more advanced species would likely have better metallurgical technologies. So with that reasoning, I would expect Galactus and the Celestials to have the capacity to create the most powerful materials in the Marvel Universe.