I think Hamonic is great, and he is my favorite Islander.
Having said that, I still feel he will top out a touch below the other two.
And having said that, McDonagh is probably my personal favorite d-man in the NHL these days, but OEL will be nearly as good and just as effective defensively (because wherever he isn't as good, he'll make up for it by moving the puck out of the zone and keeping it in the other end longer) while obviously having the higher skill set offensively.
However, I also think people underestimate the tools that McDonagh does have offensively, even though he does not use them nearly as much, and I believe that McDonagh has the potential (it won't necessarily be realized) to be a 40+ point defender in a couple of years. I believe he gets sold short in that regard sometimes; his hands and vision are better than people think and he jumps up in the play a good amount. He has to work on his touch with his passes and the accuracy on his shot. I also think that OEL, while excellent defensively, gets a little extra love in that regard from HF because he is one of the current HF golden children. I believe McDonagh is superior in his own end. OEL will close the gap, though McD will also continue to improve, and I don't think OEL will ever be as dominant in his own zone as McDonagh. As mentioned earlier however, sometimes the best defense is a good offense (puck's not in your own end, the other team can't score) and OEL will contribute to that heavily.
All things considered, OEL will have the best overall career and will be one of the very best dmen in the league for a large chunk of his career, IMO.
Edit: Obviously, I did not take into account team trophies such as the Cup because that's not always within a player's control. I answered in terms of the legacy of their actual on ice performances by career's end.
It's clearly OEL to me. He's got the talent to be one of the elite defensemen in this league. Hamonic and McDonagh both will probably become solid (first pairing?) defenseman, but OEL will be a guy that carries the entire team.