Best way to import excel/csv/txt file into MySQL

There are couple of ways you can do this. Although Imtiyaz's code does not show it, you can use an excel automation in .NET in order to create a DataTable object from the existing excel file, but this can be a rather slow approach and also it will require a MS Excel installed on the machine.
A faster approach would be to use OLEDB for this.

But yes I presume that a LOAD DATA INFILE would have the best performances, but you cannot use it with an excel file because it requires a text file. So if you would first convert your Excel file into a CSV format in C# with something like this .NET component for fast excel processing then you could use it.