im new to bitcoin and want to find out which is the best API or way for getting the bitcoin balance of a bitcoin address for any bitcoin address in C#
The easiest way would be to use the API of a block explorer. Several provide such an API and some have wrapper libraries for C#.
For example the blockchain.info blockchain explorer has such an API and has a C# wrapper as well. Consult their Blockchain Data API which explains how to get the balance (among other data) from an address. More specifically, you need to call the following (HTTP) API:
The result is a json object and "final_balance" is what you need.
If you use the C# wrapper you can find an example on how to get the balance of an address (and many other things) here.
Alternatively, you could query a bitcoin node directly. The easiest way to do that in C# would be to use a wrapper library.