I have not watched very old ones such as Mother India but I will write from the ones I know. I will use new ones more also because people find it helpful to watch. Like I am in this website wondering what would be good to watch next. I WOULD SAY:
1) SHOLAY: Definitely very good.
2) 3 Idiots
3) Kuch Kuch ho ta hai
4) My name is Khan
5) Rang De Basanti
6) Black
7) SLum Dog Millionaire
8) Taare Jameen Par
9) Rab ne Bani di Jodi
10) Ghazini