Hi,
India is really becoming more and more expensive on Health and related expenditure. Health Insurance therefore is really sensible to buy at a young age.
There is no one answer to your question, as it would depend upon the age, health conditions, your location, your additional requirements, whether you can go for a Medical Exam or not, the amount of cover you want/need.
I would therefore recommend Indians to check out the website http://www.insurancemall.in for comparing various quotes and features that you may be looking at.

Thumb Rule would be
If Age < 45 and Good Health Condition - Reliance
If Age <45 and Some pre-existing - Apollo DKV [They dont cover Pre-existing illnesses, but they would accept proposals after excluding the same]
If Age > 45 < 60 - Apollo DKV
If Age > 60 - Bajaj Allianz Silver is the best policy

When buying a Health Insurance you should take note of the following
1. Are there any sublimits as to room rent etc. in the policy
2. Has the Insurance Co. changed its TPA regularly.
3. What is the claims experience of your broker/agent?
4. Pre-underwritten exclusions if any - specially for age above 60 is very important to check.
5. Compulsory Co-payment - Many policies have a part of the expenses mandatorily required to be borne by insured - check that.

NOTE: PLEASE DONT COMPARE HEALTH/MEDICAL INSURANCE FROM GENERAL INSURANCE COMPANIES WITH LIFE INSURANCE COMPANIES LIKE LIC AND RELIANCE HEALTH + WEALTH OR EVEN TATA AIG...
The reason is that Life Insurance Health Plans are more of Benefit plans with fixed daily cash allowances...whereas General Insurance Health Plans are better in terms of RISK COVERAGE on Illness/Accidental Hospitalisations.
LIFE HEALTH covers are good as ADD On to regular Mediclaim/Health Insurance from General Insurance Cos.