The actress has made her industry comeback in the horror sequel Bhoot Returns.
She told Bollywood Hungama: "[Aamir Khan] is really committed. Before he does anything, he analyses a lot. He sees all the sides of the coin. He loves discussing one scene for hours.
"That's his involvement and shows his commitment to work. We used to love sitting around with him, me, Mansoor Khan and all, and seeing him talk was a learning experience.
"Today we don't get to see such actors, trust me. Working with all three [Aamir, Salman Khan and Shah Rukh Khan] has been a dream come true and I've made friends for life.
"If you ask me, among the three Khans, Aamir is the best actor."
Koirala recently admitted that she thinks new Bollywood films are better than older features.
It was claimed earlier this year that the film Heroine was inspired by the career of the 42-year-old actress.