Hi Ali
It's quite difficult to answer this question because the exact box-office receipts and overseas rights amounts are never fully revealed by Bollywood producers. Moreover, the value of the rupee keeps changing (usually dips). Sholay, which was released in 1975 and ran till 1980, would have made much less money than an average hit these days.
However, Hum Aapke Hain Kaun is believed to be one of the biggest hits ever -- having grossed over Rs 200 crore (crore=10 million). In its time it was a record. Then came Dilwale Dulhaniya Le Jayenge, which was an even bigger hit. In recent years, Devdas could rank among the big grossers.