Because of her delegate count, it is assumed by most in the media that Hillary Clinton is the strongest candate. But a check of recent poll averages tells something quite different. Nationally, for example, adding the polls against Trump, Cruz and Rubio, Sanders comes out 17 points ahead. In states where there are comparable polls he leads by 135 points with doubled digit leads in Illinois, Michigan Minnesota, New Hampshire, Virginia and Wisconsin. The only state in which Clinton is ahead by double digits is Pennsylvania.
Obviously, lumping the polls against the three top GOP choices is not what happens in a real election but it is useful guide to the relative strength of the Democratic candidates.