First off, PascalCasing is commonly reserved for public properties, consts, methods, etc of the class. So I would skip 2 and 5.
Second, hungarian notation is discouraged in the .NET world, so (uh, I think) 3 is right out. Assuming that's what is going on with 3.
That leaves with camelCasing and _camelCasing. I typically use _camelCasing for class variables, and plain old camelCasing for variables scoped to a method or narrower. Camel casing is the accepted standard used for method arguments, protected/private variable names and variables within a method or narrower scope.
I also like to prepend with the underscore so that my private variables are grouped in my intellisense. However, I only do this for variables scoped to a type. Variables declared within a method or narrower scope I leave the underscore off. Makes it easy to keep them separate and keep less used variables together.