Federer makes it look easy and hardly breaks a sweat ever. I sometimes get annoyed because he doesn't seem to play with his heart. Borg also had that aura of seemingly being indefatigable. Rios has already been mentioned. I also think clean flat hitters make things look easy as their motions are very simple and straight. I thought Petr Korda or Michael Stich would be a good example of that style. Stich Would seem to be doing a walk in the park while hitting aces. I don't know if there is anyone now with such a smooth style.
"Most of us believe in trying to make other people happy only if they can be happy in ways which we approve." Robert S. Lynd