What is the best data type to use for money in C#?

Use the Money pattern from Patterns of Enterprise Application Architecture. specify amount as decimal and the currency as an enum.

System.Decimal

The Decimal value type represents decimal numbers ranging from positive 79,228,162,514,264,337,593,543,950,335 to negative 79,228,162,514,264,337,593,543,950,335. The Decimal value type is appropriate for financial calculations requiring large numbers of significant integral and fractional digits and no round-off errors. The Decimal type does not eliminate the need for rounding. Rather, it minimizes errors due to rounding.

I'd like to point to this excellent answer by zneak on why double shouldn't be used.

As it is described at decimal as:

The decimal keyword indicates a 128-bit data type. Compared to floating-point types, the decimal type has more precision and a smaller range, which makes it appropriate for financial and monetary calculations.

You can use a decimal as follows:

decimal myMoney = 300.5m;