I feel like we're in an era of unpredictable predictableness. Meaning, chances are we see one of Fedal win it, but I also wouldn't be surprised if neither does and we see Cilic, Stan, or even Sascha win it. Or someone we're not expecting.
Anyhow, I'd like to see how Canada and Cincy go before making predictions. In particular, I want to see how Rafa looks - if he can keep his form up, or if he starts struggling. I also want to see if Roger maintains his high level, as there might be a bit of a letdown after accomplishing his main goal of 2017 (and the rest of his career, perhaps). Even if Roger skips Canada and goes out early in Cincy, I think he regains focus for the USO, but won't be the big favorite he was at Wimbledon.
To me the dark horses are Cilic and Stan, both of whom have something to prove: Stan that he's not slipping, and Cilic that he really can hang with the Big Four like he says he can.
I'd also add in Sascha Zverev, who I think will win one of the next six Slams before his 22nd birthday in April of 2019. Probably not yet, but I wouldn't be surprised. I see him reaching Slam-winning readiness more like in 2018 Wimby/USO.
I don't think Novak will play, and if Andy does I really doubt he wins it, but you can't count him out.
So for me the order, right now, would be:
1. Roger
2. Rafa
3. Cilic
4. Wawrinka
5. Murray
6. Zverev
7. Everyone else
(Assuming Novak doesn't play; if he does, who knows - maybe between Stan and Andy)