I'm wondering what materials would be considered the best for creating top-quality items?
For example, in real life, things like diamonds or titanium alloys are considered hard/strong.
In role playing worlds, there's Mithril and Adamantine which could both be considered 'premium' materials. Are there any others?
Update: As per comments, it would be good for each individual answer to apply to a single setting, providing a list of the materials in that setting and for each a short note on its key attributes.
If this may be an overwhelming amount of information, go ahead and limit it to a sensible amount, but do mention if this has been done.
(And since this is now a Community Wiki question, users can feel free to append extra information to any answers, to help improve them.)