In my opinion the five best movies of Aamir Khan are(not necessarily in this order)-
Lagaan - as the rustic cricketer. Aamir was amazing in his lingo, his gait, his mannerisms and all the more appreciable because of the complete changeover when Dil Chahta hai was released. Two really contrasting roles.
Andaaz Apna Apna - Who can forget this one of the great comedies. As Amar of the Amar-Prem duo. One of the finest comic timings. A collector’s item.
Rangeela - mumbaiyya tapori at his best.
Akele Hum Akele Tum - a very touching and moving role as a egoistical husband and a caring father.
Sarfarosh/Baazi - Aamir Khan in roles of a policeman. fantastic performances. Really made you feel proud of such policemen and by the end of the movies had you wishing that there were more like him in the Indian police force.
Other roles worth mentioning are Ghulam, Raakh, Hum hain raahi pyaar ke and of course QSQT.