Discussion Legendary Chests and Loot

With regard to legendary chests found at the end of quests, has anyone found that certain places have better loot than others?

For example, you might think that the leg chests in Mhoire Castle might be better than others since they are so hard to get to, and the quests there have the biggest rewards. I don't know if that is true though.

Also, if I get a key from a quest in Mhoire Castle can I save it and use it at the end of some other quest? It looks like you can do this.

Where have people found the best loot chests to be?