The pop singer which I like best is MJ.
My favorite pop singer is MJ.
意思是：我最喜欢的流行歌手是MJ.
这里把The pop singer放在了句首作为主语,那后面肯定就要用一个定语从句来修饰.谓语是is,宾语是MJ.所以就用which I like best来修饰The pop singer.