Which is best sunblock available which protects whole day? Is it necessary to buy new sunblock every year?

Zinc / Titanium. Sunscreens work via 2 possible ways: 1) chemical blocks that breakdown the uv rays as they pass through the skin and 2) physical blocks that prevent the uv rays from getting to the skin. I recommend the physical blocks (zinc and titanium) because they have a very good safety profile and work very effectively. If you are going to be in the sun - use a minimum of SPF 15 - follow product directions.
Must reapply. Sunscreens are designed to be stable about 3 years. Look for products that have micronized zinc oxide and titanium dioxide. Broad-spectrum physical blocking agents protect against both uva and uvb rays without harmful chemicals. Use a minimum spf 30, (remember spf applies only to uvb rays), apply 20-30 min before you leave the house, apply 1 ounce for the entire body, and reapply every 2-3 hour.