Specialising in MS windows, Linux or Mac OSX?

I currently work in IT support in an environment which uses a wide range of OSs. Windows, linux and Mac OSX. I provide 1st and 2nd level support for all these OSs. I am at a stage where I need to specialise but I need to consider which way I want to go. I enjoy working with all the OSs, so I do not have a preference myself. But I want to specialise in an area which there is greatest demand and greatest job prospects. Can anyone give some advice. Thanks