Gronk by consensus this year is a top 10 player in the league and a top 2 non-QB. I think the majority of observers would halve those figures, making a top-5 player and the very top non-QB, ahead of Watt in this terrible Houston season.
I don't think there was quite the same view of Moss, record-setting season notwithstanding.
On the other hand, Welker was a Pro Bowler and Edelman probably won't be.
With that as background, I'd say that the view of Gronk as being ahead of Moss is probably right. Moss made a huge contribution with his plays, but he didn't twist entire defenses into knots the way Gronk seems to.
On Welker vs. Edelman, I have to think that if Edelman were used the way Welker was he'd be as good. His much greater catch radius outweighs his dropsies; he's always been yet better than Welker at YAC; and I think he's become an outstanding route runner by now, which is the only area where Welker could be potentially ahead. (Except, of course, for backup placekicking ...)