There shouldn't be any prefixes for anything because you shouldn't be using Hungarian Notation at all. Hungarian Notation was created and gained popularity at a time when there was a small number of types and no IDEs with Intellisense. Hungarian Notation no longer serves a purpose so don't use it at all. If you do insist on using it then you can use whatever prefixes you like because there is no such thing official Hungarian Notation. If there was then everyone would still be using "sz" for Strings instead of "str" in manay cases or just "s" in many others. Three prefixes for String which is one of the most commonly used types? Yeah, Hungarian Notation is a great idea, especially when any decent developer should be using variable names that make it plain when a variable represents a String without any prefix. If you see a variable 'name', would you think that is was a number? As you can tell, I'm not a fan. It's not used anywhere in the .NET Framework so there's no reason to use it anywhere in your own code.
Saying HN no longer serves a purpose is being short sighted IMHO. Sure HN is unnecessary in .NET languages in the confines of the Visual Studio IDE, with all the nice intellisense, but a lot of programmers also need to work in less strongly typed or untyped language as well. Having a consistent notation across languages makes it easier to read as a whole. Also the fact that it is not strictly standardized is not really an issue, as long as there is consistency in notation across a team. And even within VS, HN helps analysts and QA's to understand pastes or printouts easier.
The bottomline is pick up a pattern and be consistent with it.