By devastating, are we saying most effective, most explosive, most unique, most memorable or most overpowering? Of the choices, I immediately narrowed it down to Agassi's Return of Serve, the Sampras Serve and the Goran Serve. I have watched tennis obsessively since the late 80s and have watched clips and matches of just about all of the greatest players going back to the early 60s. Sampras and Ivanisevic had serves that no one could handle. But there are other players who have tremendous serves. Karlovic, Roddick, Krajicek, Isner, Phillipoussis etc. Of the serves, i would say Sampras was most effective, because of his disguise, control and variety. Ivanisevic had a lights out first serve. Basically unreturnable, if it was in.
I am trying to be as honest as I can about this question. Using the criteria of the shot(game style etc.) being effective, explosive, unique, memorable, overpowering and (as the thread states) Devastating, I would say that Agassi's return easily beats them all. I've seen big serves. I've seen (overall) better service return games than Agassi. I've seen more explosive groundstrokes, better touch, better ball control, better strategy and just plain better overall tennis talent than Agassi.
However, I have NEVER seen anyone return a 145 to 150 mph serve at the shoelaces of a massive server, except for Agassi. I have seen him do it to Taylor Dent, Roddick and Karlovic. Yes, Federer can return a serve with that pace. But not with the kind of velocity that Agassi did. Look, its really more of a parlor trick. I think Hewitt, Connors and probably Federer had better(or more consistent) overall service return games, but the ability to take the hardest serve in the world, on the rise and put it back over the net for a clean winner or at the feet of the server (who has not even finished his follow through on the service motion), is the most devastating shot I have ever seen in tennis. I remember seeing Taylor Dent's expression after Agassi cleaned up his biggest serve of the day. He really looked like he had seen a ghost. Imagine being able to serve 140+ mph from age 16 up. You are finally on the tour and a 30 something Agassi returns your biggest serve for a clean winner. If no one had ever done that before, I guess it would be very hard to believe it. Or accept it.