It really depends on what you are doing. The most popular RGB profile used as a source is Adobe 1998 but there are a variety of options. Each different RGB source profile will perform differently. One concern is that whatever RGB source profile you use be sure that it is 100% efficient to L*AB. You can see a chart of profiles that are 100% efficient to L*AB at
www.brucelindbloom.com
www.brucelindbloom.com