No particular order here: (Mainstream Only)
Dil Chahta Hai: The simple tale of love, told through the experiences of three friends, this movie raised the bar of movie making for ever. Slick production, amazing performances and music and a great eye for detail ensure you can keep watching DCH.
Hum Aapke Hain Kaun: The mother of all soap operas, this feel good movie did exactly that made you feel good. Eminently hummable tunes, elaborate sets are rituals and plethors of emotions turned the tide for using entertainment as medium of showcasing lifestyle.
Sholay: One word: Blockbuster. This is the first and the truly big blockbuster which has fans across generations. And you tend to pick characters you like and they become immortal. The cult of sholay.
Satya: The ultimate gang flick. It tracks the grim realities of the underworld which is so entwined in our normal lives that you wouldnt probably realized it. Hard hitting performances and a raw urban energy made Sathya the benchmark of all such films
Zanjeer: Rise of the angry young man. The tall brooding and upright officer was the beginning of the rise of the Big B. Add to that the innocense of Jaya Bhaduri and the presence of Pran and Big B as a phenomenon begins.
Munnabhai: I dont know whether this genre will be repeated or not. But this series of movies throws back ot Raj Kapur times (excpet the immense sorrow :D ) where you have good people and bad people who are not assumed to beyond redemption. Humurous street lingo and good chemistry between the actors ensures that this might be a 'formula' that might work in future too.