What is the なのは used in "なかでも にんき なのは ポケモンどうしを たたかわせて".
I know にんき is a na-adjective but i don't understand the use of なのは.
の is a noun with general reference. The na-adjective attributively modifies it.
は is the topic marker. So you get
人気なのは "as for the thing that is popular".