Dilip Kumar, no doubt is the greater thespian, but the Big B is also in a class of his own, unparalleled by anyone!
You can see shades of Dilip Kumar in Amitabh Bachans acting so there lies no doubt as to who is superior.However, Amitabh is probably much more popular because of the open media now which has been advantageous to him..
Media does play an important in increasing popularity of Bachchan, but the various shades of roles he has played also added to his fan following. He has played larger-than-life heroes as well as common person with shades of villainy inside him. More importantly, he played the roles of angry young men out to avenge the unfair system, something the youth of that era completely identified with. This made him more desirable.
Have you seen his 'Saudagar' with Nutan playing his leading woman? Its not very well known movie, but I think his role in that movie is one his best ones!