Jason Day seems to always have some sort of health issue going on, and because of this I don't see him being consistently winning long enough to remain even where he is right now.
Rory McIlroy is a head case IMHO. Greatest talent for the game, but he allows distraction and outside things to influence his game. I don't see Rory fulfilling his potential in the long run, which if he went based solely on his talents, he could probably win 20 majors. But he's screwing around with sponsors, equipment, women, and allowing a horde of hangers on to influence his every whim. Rory has maybe five good years left to do something spectacular, but ultimately I see him crashing and burning. He says all the right things, and has great PR for the most part, but he's trying to be all things to all people, and that will make him borderline suicidal eventually.
Rickie Fowler seems to not only have the talent, but the killer instinct for a long term run. It took him awhile to find that next elite level that only a few golfers ever find, but I think if he stays away from skanks and low class idiots (like Bubba) then the sky is the limit. I can see Rickie finally winning a major (or two) this year, and he could be in the conversation for #1 at the end of the year.
Jordan Spieth is still a mystery to me. He's already shown incredible talent and a bit of luck to win two majors basically right out of the box. But his putting stats from early last year indicate that this is not a long term thing. You don't make those % of 20+ footers for a long period of time, which is how he was winning early last year. I look for him to possibly struggle to compete at the majors, even though he'll probably be towards the top of the leaderboard. Can he continue to win like last year? Possibly. But unless he finds a way to continue to putt like he did last April and June, then he'll be mixing in a few MC's with some top 25 finishes, and we may see him come down to earth even as early as this year. The jury is still out for sustained success from him. Show me this year you can come even close to what you had last year, and I might buy in.
Of the above four golfers I see the biggest upside tragectory in Rickie Fowler, with Spieth, McIlroy and Day coming next in that order respectively. Fowler impresses me most out of the four though.