Which hashing algorithm is best for HMAC applications? "Best" is a combination of strength and availability of libraries to generate a hash.
The server-side stack is all Microsoft, specifically .NET 4.5. The clients are potentially any client out there but likely .NET also.
So HMAC is pretty standard, and available in Java, .NET, PHP. From a quick look at the Java and PHP docs if the language supports a hashing algorithm if appears to support using it in HMAC generation. Given that, the strongest is going to be SHA512.
You should also look at the NIST guidelines and checked algorithms.
