Some people said Zheng had it
Somebody think Serena has it now, somebody Hsieh.
I guess Na Li's RoS one of best.
Hsieh had the best RoS relative to her ranking one year ago. Against the much better servers her RoS is rather average.
Zheng has an incredible RoS reaction time, allowing her to attack kick serve's despite her shortness. She can soften the serve of some serve-based players rather well (Stosur, Petrova, S.Williams), but that is not nearly enough ...
S.Williams has an incredible court presence, making it almost impossible for any server to keep calm. She mercilessly punishes week serves, but her RoS against good serves are not extraordinary. They were, however, much more consistent than those of brainless ballbashers with similar power on their RoS.
Li had a typical BBB RoS. She made quite a few good RoS winners because of her agility and the high tension strings, but she usually UEs at least twice as many, and against 2nd serves too. She was about the worst returner in the top 30 in recent years.
Last year, Azarenka had the best RoS. Deep, consistent, perfectly opening the rallies for her. And she did it against almost all kinds of serves.