The most devastating shot of all time should NOT be surface dependent. That rules out the Pete Sampras serve. Karlovic, OTOH, served 77 aces on clay in a SINGLE match. Now THAT's a devastating shot.
I agree; Gonzo's FH is incredible. When it's on, he is hard to stop. Somehow, though, he hasn't won a Grand Slam - which is why he isn't on the list. Even though it is extremely devastating, it hasn't been devastating enough to win him a major. And if you haven't won a major, then you can't really be considered for this kind of poll (best shots ever), since winning a GS is the ultimate proof of a devastating shot (since normally it is one shot that contributes more to the player's victory than the others). Still, Gonzo's FH is among the best in the world, no doubt.