Choosing a geocoding service should be a decision made according to the application requirements and I am unable to determine which service to use. You should be able to find such services trough public materials (e.g. List of geocoding systems).
You can find attached a simple project that demonstrates the Bing services integration. Note as a custom solution the behavior and the expected results should be further implemented as per to application requirements.
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I'm a newbie to webservices. Could you please explain how did you use MapQuest's Geocoding API in your application. I have a similar requirement- mark the locations in map based on the address in datagrid. Thanks in advance.