不是的.
one of 后面加形容词的话不是（注意：不是）只能加最高级
你的例句：
he is one of _____singers
可以填
the most famous
也可以填
the more famous
再举个例子：
John is one of the best swimmers in his class
Mary Brown is one of the better writers in this group
Jane and Michelle are two of the better dressed girls in the room
都对.
只填 famous 或 the famous 的确是错的,
因为 这个句型要用 比较级或最高级,the more ...或 the most ....
我是加拿大人,前海外英语老师.