The cheapest flights are generally to Delhi. There are a number of other international airports in India but Delhi is generally the cheapest and most traveled. Technically there are international airports located in the following cities: Kerala, Cochin, Bangalore, Chennai, Goa, Hyderabad, Mumbai, Kolkata, Ahmedabad, Guwahati and Amritsar however most western flyers arrive in Delhi and for this reason it is typically the cheapest.

It makes sense to fly into Delhi if your trip is centered in North India or covering a lot of the country. If you are flying from an Asian country it may be cheaper to cheaper or more convenient to fly to Kolkata if you plan to be in the north east (Kolkata, Orissa, Darjeeling, West Bengal etc) or Chennai if you plan on seeing south India.

Cheap flights to Delhi can be found easily. Check my cheap tickets to India section for some help.