I am trying to develop a java application that displays : Google maps, Openstreetmap and Bing Map Data. So which is the best SDK to use in this contet Geotools or uDig ?
It depends on how advanced application you will be creating. I recommend downloading uDig, and starting the application. If it looks close to what you need, then go for uDig. If you want a more low-level API, then you should select GeoTools (which uDig is built on top of).