"Superstar" is a relative term and as long as media and PR exists, there would always be some actor who would be referred as a "Superstar". Whether you agree with it or not is another thing.
Currently some people only consider Salman a Superstar, but then there are some who also include Aamir with him in the list. Then more people come and they add SRK as well, considering only the Khans as the last bunch of Superstars in Bollywood.
Include more people and you would find them adding Hrithik along with the Khans. Ask more people and they would call Akshay a Superstar as well. Dig more and there would be people who would also consider Ajay as a Superstar.
In my opinion all of the above mentioned actors along with Ranbir are currently the only Superstars in Bollywood, but at different levels, although that's just my opinion.

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