Helmets damage your spine, just below your head, in rough road conditions, due to its weight. Helmets available in India are mostly bulky, Only lightweight helmets available in India are the safety helmets used in industries which are not really designed for road use. Its high time for companies to make some lightweight helmets available in India.
Because of the tropical weather, most of the roads in India are rather dusty. So the visor almost never stays clean. This causes problems particularly at night, especially when there are light showers that diffuse the headlights coming from the opposite direction.
So a reasonably good helmet must be lightweight. It must have a visor that is easily cleanable and it must break on impact but not into sharp pieces, which may hurt head or face seriously.
The choice of helmet is also largely influenced by your budget. If your budget allows you can go for Snell and DOT certified helmets but if you are short on budget, you can go for ISI certified helmets from brands like Studds, Steelbird, etc.
Choose by the quality, durability, visibility and comfort of the helmet.