I think Han Hong is best because she has the biggest vocal range. The other 2 can only sing high, because their talking voice is high too. Han Hong can go very high or very low. But 李谷一 is impressive at her age.
不正当竞争! I think they all are great, I personally prefer Han Hong 韩红, but as you got older it is harder to sing, so it is unfair to 李谷一 . 宋祖英 is very good too for her style, very Chinese sounding.