As a human being, Xiaozhao's the 'nicest'. But these are characters in novels. I don't read novels for 'niceness' - I can get enough of that in real life - but rather for what's interesting. Zhao Min and Yin Li are interesting in the sense of being fun, but they also happen to be typical Jinyong male fantasy-women. They make for mediocre literature. Zhou Zhiruo, on the other hand, is about as much fun as being stabbed in the chest with a sword, but she's somewhat better-written (until the last chapter of Yi Tian, when Jinyong - characteristically - undoes all his work by reverting back to his indulgently sentimental self).
So I'll vote for ZZR. Because the way to a man's heart is through his rib cage.