Sampras had the most smooth and fluid serve I've ever seen and I think the best serve of all time. The fact that his serve still holds up after not playing at all for 4 years just shows how natural it was for him.
Federer should not be getting so much attention for his serve. It's good, but he'll be the first one to say that there are guys out there with better serves.
And Roddick's serve IS great and there's not a single pro that would disagree with that. If you say his serve isn't that great, then his ground game MUST be great, because he's been at the top of tennis for a while. It's not efficient for most people, requires a lot of energy, but he's a twitchy kind of guy and I think it's natural for him.