Let me give my opinion.
I think Rory's being really underrated. We can all give our own opinions, but the stats don't lie. Rory's been elite compared to his peers over the course of his career. He's streaky, we can pretty much conclude that these streaks will happen. He will have years where he's the best player in the world, and years where he's very average, but I don't think he's all of a sudden done, and I think he's due for a good season.
I'm slightly worried about DJ's putting. It completely fell off after the Masters last year. It was back to typical DJ. I know he just won, but I don't expect him to be one of the few best players in the world this year. He never has a bad year, he wins like every year and contends in big tournaments, but I don't think he's going to win 3, 4, 5 times. I think maybe he gets another win, maybe not. I don't think he's going to have a big year, although I don't expect a terrible year. Safely top 10 this year, but I don't think he'll be one of the sport's main characters in 2018.
I actually expect a big year from Rahm. I think he's going to win many times, and contend in a major, maybe a few majors. Pretty much everyone fails to win a major the first time they contend. Occasionally some don't, but I would expect the first major he contends in he won't win. Still, 3 or 4 wins, and one of the best players in the world this year is what I think he'll do. Is he better than the best players? No, and I didn't think Thomas was last year. You gotta prove yourself. Rory has 4 majors, Jordan has 3, DJ wins every year. Prove yourself for more than one season.
I think Thomas will have a down year, I think he'll win an event, maybe two, but he won 5 times last year. I also don't think he's shown such great consistency yet. His good golf is really good, but he doesn't really grind out rounds too well yet to make the cut when he's not playing well, and then back door a good finish. I think he'll have a let-down year, and I'm not convinced he'll ever show consistency.
Jordan's seasons are always very reliable. 2016 was a "down" year, and he still won twice, nearly won a major, ranked top 5 in like every big category, so his "down years" are better than others down years. And an average year like last year is still first in OWGR points gained, including a major, so I would say you can reliably say Jordan will put up one of the better years of any of these golfers. If the putting comes around, he'll probably be the best player this season. If not, he'll likely still be one of the best in the world.
I'm not convinced anyone else is breaking into the top level. Day could have a bounce back year, but I don't expect him to be back to the top of the sport. Matsuyama could win some events, but unless he fixes the robotics in his game, its a problem. Rose is solid. Good enough for a great year? I don't know. Fleetwood is the player who's game I can't get a handle on. He's really good, but I can't figure out why. He's an excellent ball striker, but what else is he really good at? He gets results though, been very impressive. Hossler, Schauffele and Cantlay aren't ready to make that level of breakthrough, in my opinion. They'll probably have good seasons, but I don't expect them to be top players this season. I'm hoping for a big year from Patty Reed, maybe Berger, as well. Tyrrell Hatton is another who I think could have a very good season.