He the most versatile actor in Indian Cinema. Switching roles and adapting to it isn't an easy task. He has probably took up every project which involved a different character. While most of the actors are picked for a certain demographic audience, Aamir Khan is an actor who caters to everyone's taste.
He was brilliant in Andaz Apna Apna, Jo Jeeta Wohi Sikandar, Sarfarosh, Akele Hum Akele Tum, Dil Chahta Hai, Taare Zameen Par & 3 Idiots. I have seen most of his movies since Jo Jeeta Wohi Sikandar, barring Earth and Mangal Pandey. Not all of his movies may have massive commercial success, but his performance is appreciated in every movie. Ghajini may have been a sad movie, but he went through a lot of physical training to prepare for the role, results of which were evident.
An actor is not just judged on the basis of the overall success of his movies, but also for his individual contribution in it. Take Hrithik Roshan's performace in Mission Kashmir or John Abraham's role in Karam. Both of these actors have a great advantage of looks and physique, a criteria, which is worshipped by Indian cinema for some reason. Most of their movies are utterly rubbish and one would easily loose faith in their acting skills due to lousy scripts. But it's due certain roles that they perform, it really makes us think what they would be capable of, provided they get a proper role.
Aamir is consistent is his performance and that is something which is very rare in actors these days. The only other actors I appreciate in that respect would be Amitabh Bachchan & Sanjay Dutt. I would also appreciate Shahrukh Khan, provided he breaks free from Yashraj banner for sometime and focus on better film subjects. Aamir has always stood out of the rest for experimenting with his roles and he is good at it. This is just my opinion. I see talent in him, as I do in few others.