Jack Kramer, as shown here (see 0:21 in particular) in the 1947 Wimbledon final against fellow American Tom Brown, had a smooth and powerful service motion, but he was limited by rules in place at that time.
Prior to 1961, the International Lawn Tennis Federation prohibited players from having both feet off the ground while serving. The rule obviously restricted their extension and racquet-head speed because they could not tap into the full potential of their legs.
Some players remarkably managed to rack up big ace counts despite that limitation, with Ellsworth Vines famously hitting 30 aces in just 12 service games in the 1932 Wimbledon final against Bunny Austin.
Vines's serving prowess inspired Bunyan-esque tall tales, including the following exchange cited by Palm Beach Life magazine:
Umpire: “Go ahead and serve, Mr. Vines."
Vines: “I already did.”
Umpire (to Vines's opponent): “Did Mr. Vines serve?”
Opponent: “I don’t know. I wasn’t listening.”
Vines was the Babe Ruth of tennis, blasting aces when few others could, but the brief Kramer/Brown highlights above, or these from the 1934 Wimbledon final, indicate it was rare prior to 1961 for the serve to be a devastating weapon.
As for Vines, I'm skeptical that the same serve whizzing past the 5'9" Austin would be untouchable by bigger athletes like Roger Federer or Novak Djokovic.