Oh I think Fed is US Open favorite for sure, but Sascha is getting into servebot territory with enough return to make him a threat. Think of what a grinder Zverev was much of last year and now he's going to start mowing through the draw with quick match times at the majors. That play style gives him so much more effective stamina than a player like Thiem with a physical playing style. I really think we could see Zverev going deep at majors all too suddenly.
I am troubled by the Zverev return game on hard courts this year. Last Fall Zverev had a stunning run of returning starting at the US Open. I don't have games easy, but tennis abstract had him from US Open through to his defeat by Ferrer last Fall with 40.6% return points won which is stunning and much better than 2017 at 35.6% or all of 2016 at 37.1%. Some of this may be the slightly faster court conditions this year and he simply may have had the luck to not hit a lot of big servers in the Fall. 20% on return games at this young of an age is ok, but he must improve because once he hits 25 or so the return game will start dropping and 20% on return games is marginal for a slam winner as I'm sure you're aware (Roddick

.)
Grass may have the same stat anomaly as a short period like Fall 2016, but Zverev did very, very well. Zverev was about fifth on first return stats winning 31.8% (Fed last year 28.0% and 2015 was a top notch 30.5% of points. Zverev broke 24.5% of the time in 2016 on grass, Fed 18.7% in 2016 and 21.3% in 2015. Based on his hard court return progress (pretty small) I doubt Zverev improves and he probably drops as 24.5% on the high side for only 8 matches.
Zed could easily get in the 91%/21% range on grass games this year would be my guess. Fed in 2015 was 95.8%/21.3% on games. If they meet on grass, it would boil down to who can break who. Zverev is certainly going to give most anyone a match on grass this year.
On clay impressive stuff by Zverev as Raonic and Isner had been hard to break this year.