Ulaganayagan, Megastar & Superstar...
Calling all of them simply superstars will make them look so small... Their titles bestowed upon them by the fans & trade justifies their super stardom & makes them stand apart from other bunch of "superstars".
I mean Rajinikanth directly got the Superstar tag whereas Kamal Haasan & Chiranjeevi worked harder to earn the bigger titles... Kamal Haasan from Sakalakala Vallavan to Ulaganayagan. Chiranjeevi from Supreme Hero to Megastar. The hard work, fan following is all there to witness the perfect transition. That's why calling them "superstars" makes it look simple.