iggest 形容 characteristics 是chinglish了,只懂英语不懂汉语的为理解为（体积上）最大的.
characteristics不加s,因为是最大的那一个.
each,each one都不能说有错,但是都有歧义,each 什么你没说,each deck?each card?each suit?一副/张/套花色
我会这么说：
The most prominent characteristic of a deck playing cards is that each card is different from the rest.
The most prominent characteristic of the playing cards is that every card is unqiue within a deck.
The most prominent characteristic of the playing cards is that no two cards in a deck are the same.