Most likely OU Pokemon, although it IS hard to tell which are the most popular, considering everyone has different opinions, personally I have 5 favourite Pokemon, including Reshiram and Scraggy. Of course not everyone would agree with what I think, just like I probably wouldn't agree with other peoples. Yet as long as people can respect others points of views, that's all that matters, and so all of us can be happy.
Hope I helped! :)