I think "most rapid" is heavily subjective. A developer with many years in VB will likely be fastest at prototyping in VB. A Java developer in Java. Ruby in Ruby. The "most rapid", then, is going to be heavily skewed by the assets (code libraries, developer experience and tools) you already have in house.
What you define as a "prototype" also heavily affects things. Is a set of pseudo-working screen shots mocked up in Flash to have some clickability for navigation enough? What is the required feature set and what is the target audience for the prototype?
As you can see "best" is going to vary pretty widely. It's probably close to certain that the language will be high-level and the IDE tools are going to have nice UI designers (assuming the prototype has a UI). If you have a lot of DB work, then database wizards that do the SQL grunt work for you will save time and generate reasonable, if not optimized, objects. The platform would likely be whatever platform the prototype should be for - after all prototyping a Windows app under Linux or a Symbian app under Palm OS probably won't give you too much benefit.