“好朋友”和“知己”的差别是什么？

好朋友：Good friend 

知己：It literally means "A person know/understand you really well".  I think it is closest to the word "soulmate" in English.

Most of the time, Your good friends can understand you better than other people. So 知己 is usually among your 好朋友。

However, 知己 is not necessary be your 好朋友. Because sometimes, you might meet people who can understand your thoughts and feelings well, even you have known each other just for a few days.   

But if  you use 知己 to describe the friendship with the opposite sex. 知己 will mean as the same as what you had written.  in Chinese, we often hear the sentence similar to "比朋友更近一点，比情人更远一点。 "——“友情以上，恋人未满"

“好朋友”和“知己”