Mystical fights are hard to compare because each era has a different style, different weight classes and formulas for getting down to weight. Considering what Robinson did in his era, I'm sure he would have matched what he did had he fought in the 80's. I don't think anyone's resume in the 80's compares.
He would beat them all, including Hagler. Whilst Hagler is the greatest middleweight in my view (in terms of accomplishments), Robinson at his best at MW could take Hagler. Hearns would definitely give him the most trouble. It's the slick boxers that would trouble Robinsons more than the come-forward fighters. That range and power is difficult to overcome, but if anyone would do it, it would be the real Sugar. Ray had dynamite of his own in both hands, and he would probably lay Hearns out late. Of course, it would've been near impossible to go through undefeated amid such talent, so I'm sure he drop a match or two on a "down" night. Though with much larger gaps between fights compared to the 40s and 50s, maybe he wouldn't have those down nights he experienced at MW, when he was aging and still fighting so frequently.
As to the question...I could see Leonard maybe taking one fight in three fight series, and Hagler splitting a couple of fights. I think Robinson decisions Duran, scores a late round stoppage of Hearns after being behind on the cards, and grinds out a late round stoppage of Benitez.