Emblem is a symbol used to describe a family of Gear (for example: Nature, Ice, Fire, Myth, etc).
Emblems include Gear from Rare quality and above. All pieces of Gear that belong to one emblem are similar in art style. Every Battle Event have one promoted Emblem each week. Gear marked with this Emblem can be inserted into the multiplier tab to improve hero's statistics in the Battle Event.
There is no "best emblem" - all are unique and different. There are many sets of the same Emblem.
To read more about emblems, check out our blog.