Your analysis is not purely unbiased. Some facts are logical but to say Rajesh Khanna's performance as monotonous and ignoring him as truly great versatile actor in comparison to Amitabh, Sanjev Kumar, Amir Khan or Balraj Sahni is most unjustifiable. In your another posting under some other topic of this Forum, you have also included the name of Dilip Kumar, Dev Anand, Dharmendra also in term of long running stars (Merathon word you have used).Comparing the 2 stars Rajesh Khanna and Dharmendra is like comparing chalk with Cheese.Dharmendra was basically a he-man who pioneered the role of Indian film Industry's first he -man.Though he did some romantic leads he basically acted as an action heroand i scenes of vengeance he was a champion.Rajesh Khanna basically did romantic leads and never played an action hero like Amitabh Bachan,or Dharmendra.
In pure talent Kakaji is ahead of Papaji(Dharmendra) and had a more natural style.However he often got carried away in projecting his own star image and ultimately we saw Rajesh Khanna ,the star and not the actor most of the time.(Monotonous stye)Only in Anand,Amar Prem,Do Raaste.Biwarchi Bawarchi or Safar did we experience his true prowess in character roles.True he was the first superstar,a jesus Christ in his days like no one before or after but after that period he could hardly mantain his stardom,unlike Amitabh Bachan,DEv Anand ,Dilip Kumar or Dharmendrawho were Bolywood's marathon men.It was more that people wanted a character like him.Dharmendra stood up for almost 3decades and was effective in comedy and character roles like in Satyakam,Anupama,Chupke Chupke and to a certain extent in Sholay.It is a remarkable achievement that an action hero could give his in romantic and idealist films.
Ultimatey both could be called graet stars but perpaps not truly great versatile actors like Amitabh Bachan ,Sanjeev Kumar,Amir Khan or Balraj Sahni.It might well be that they were hardly given the oppurtunity to enact true charcter roles and basically cast in a stero-typed manner.THe pleasure the 2 men gave us is unforgetable.
First of all Sir, Rajesh Khanna has still maintained his most dignified position as the First Super Star of Indian Cinema and as an class Actor in the hearts of million film viewers for the last four decades only owing to his exceptional versatility. Just compare him in Amar Prem and Prem Nagar; Red Rose and Dhanwaan; Mehabooba and Anurodh, Avataar and Amrit, Rajput and Janta Hawaldar, etc. His super versatility will automatically apparent before you.
Secondly in your selection list, the fact is that Amitabh, Dev Anand and Dilip Kumar are the real monotous or the most typed performers. Sanjeev Kumar and Balraj Sahni are the real actors but they did not have any star value any time. They were only actors. Amir Khan can not be compared with any of them, because he mostly appears maintaining a long long gap and his career duration is far far less than everybody in question. But it is only Rajesh Khanna, who established himself as a most versatile actor despite of his historic Super Star Image, as he always appeared mostly in author backed roles with different theme and characters (simultaneously in about five six films in a year). Moreover, in similar type of characters too, he has shown his tremendous versatile performance, because his performance directly comes through heart and in emotional scenes he is deeply involved with the characters. His facial expressions are also uncomparable. So Sir, be logical and unbiased and compare the standard and capability of characterization, which is the most significant fact for long term impact.