According to Sparknotes, these are musts

• Kubhi Kushi Kabhi Gum
• Kal Ho Na Ho
• 3 Idiots
• Salaam Namaste
• Bend It Like Beckham
• Bride And Prejudice

According to the Indiantimes, the 25 must see bollywood movies are:

• Black (2005)
• Lagaan (2001)
• Satya (1998)
• Dilwale Dulhania Le Jayenge (1995)
• Qayamat Se Qayamat Tak (1988)
• Mr. India (1987)
• Jane Bhi Do Yaaron (1983)
• Arth (1982)
• Deewaar (1975)
• Sholay (1975)
• Bobby (1973)
• Garam Hawa (1973)
• Pakeezah (1972)
• Anand (1970)
• Padosan (1968)
• TeesriManzil (1966)
• Guide (1965)
• Sahib Biwi aur Ghulam (1962)
• Mughal-e-Azam (1960)
• Pyaasa (1957)
• Mother India (1957)
• Do Aankhen Barah Haath (1957)
• Devdas (1955)
• Do Bigha Zameen (1953)
• Awara ( 1951)

Best bollywood movies of 2008, according to realbollywood.com

• Jodhaa Akbar
• Singh Is Kinng
• RNBDJ
• Fashion
• Dostana
• Ghajini
• JTYJN
• Race
• Rock On
• A Wednesday

Best Indian Movies(Bollywood), according to recipedelights.com: comedies

• Andaz Apna Apna
• Angoor
• Chalti Ka Naam Gaadi
• Chashm-e-buddoor
• Chupke Chupke
• Dil Chahta Hain
• Golmaal
• Hera Pheri
• Lage Raho Munnabhai
• Munnabhai MBBS
• Rangeela

dramas

• Aandhi
• Abhimaan
• Aitraaz
• Amar Akbar Anthony
• Anand
• Aradhana
• Bombay
• Joh Jeeta Wohi Sikandar
• Sarfarosh
• Umrao Jaan (1981)
• Yaadon Ki Baaraat

romance

• Chandni
• Dil Hai Ke Manta Nahin
• Dilwale Dulhania Le Jayenge
• Hum Aapke Hai Kaun
• Hum Dil De Chuke Sanam
• Kabhi Kabhi
• Kuch Kuch Hota Hai
• Maine Pyar Kiya
• Qayamat Se Qayamat Tak
• Saathiya
• Sadma

action

• Bandit Queen
• Company
• Sholay
• Vaastav

thrillers

• Bhoot
• Ek Hasina Thi
• Jewel Thief
• Kaun