Learning is on a spectrum. Depends how far you want to go. There come a point where it's easier to use a different language. Ya dig?
Just guessing from the context of the question that you have to learn one and it doesn't matter which. Maybe for school or some other project.
Well for starters what are you trying to accomplish. Silverlight for example is for web applications while objective C is the language behind apple computers. You need to make a distinction between the client(computer), a moblie device, and the web of what you are trying to accomplish on which one.
You will only use objecitive c on the iphone and imac so that's prob not what you are looking for. .Net is the architecture behind windows operating systems and has one of the most logical and easiest languages to learn. Java is def harder to learn than .net. If you are trying to do something on the web: ruby is a good way to create small static applications, silver light is much more powerful and a far more developed language.
Let me also say that i am not familar with Fring or Smalltalk.
Hope that helps!