I would look here:
http://www.atpworldtour.com/en/stats/service-games-won/all/all/all/
I would check career and by years, then by surface. Take into account that when you compare the 1995 to 2015 the game has changed.
At the top you have the guys with the best serves TODAY, which has to be taken into consideration. Modern rackets put more speed and more spin on the ball, so today you can serve harder AND with more net clearance. So to convert what the guys were doing in the 90s to today I would had a couple percentage points.
That would put Sampras up around Raonic in winning games, and I think that is fair. He edges out Fed.
When you go to grass Sampras tied at 93%. Again, older rackets, older strings, but he is barely behind Karlovic and Isner.
So I would put his service game up with the best of the best. He rivals the servebots. However, Pete had amazing weapons to back up the serve. So his serve itself can't compare with Karlovic, who gets more free points because of the insane speed, which comes largely from his height.
The same is true of Fed to a large degree, but I do not think his serve is as intimidating. It isn't as big, even with the new equipment. He is less likely to hit a second serve that really is a first serve and ace with it.