That's a difficult one, not least because it was the beginning of the period when I was most intensely involved with politics so I can't just view it as history. Robert seemed to be even more of an idealist than his brother, and would therefore probably have disappointed the young liberal left because of the compromises he would have inevitably had to make had he won the election. Withdrawing from the Vietnam conflict would have been his biggest problem: demanded by his supporters but difficult to do at that point without making the US look weak. And it is hard to see him having been able to engage with China as Nixon/Kissinger did because domestic opposition from the right would have made it impossible. Conversely, Watergate wouldn't have happened, and the corrosive effect that had on Americans' attitude to politics for a generation might have been avoided. He might have taken forward the civil rights legislation passed by Johnson leading to a more equal society, more at peace with itself. One of the features of getting older is recognising that every politician is going to disappoint you, and most of the ones you most hated in your youth had at least a few redeeming features.
The oddness about the Kennedy brothers (the ones who survived the Second World War anyway) is that the one who was just about directly responsible for the death of an innocent person was probably the nicest of them.