You should ideally use the largest profile you can - largest meaning the widest range of colours, usually containing brighter and more saturated colours. But that all really depends on what you intend to do with your work:
If your camera and printer support it, use Adobe RGB.
If you intend to display your work on the web, use RGB.
Most monitors and browsers can only display in RGB, and working in Adobe RGB and then saving for web can lead to washed out colours as already mentioned.