Eh..... not really high on any of the 4 as actresses, but Nixon is the only one among those I can see where she can come up with a few interesting things. She could be like a Hope Davis if she were a film actress.
This is a different case than with "Friends" and "Girls" where there's a clear standout among the cast. Kudrow is way above her cast members, and Zosia Mamet way above the her's.
Anyway I'll give the edge to Samantha as much as I love Miranda's sarcastic timing. Samantha is such a difficult role to be made likeable and Kim Cattrall was faultless at it. Before this show no one would even imagine the popularity an extremely sexual and unapologetic character like Sam Jones could gain.
Nixon was good, she was obviously much better than those other two. She got a lot of the respect from people when the show was on it seemed almost on account of her being the 'serious' one, the more respectable one, and she was (and is) a fine dramatic actress to be sure, with a great dry wit.
But Cattrall was a fucking force of nature. That is one of the great TV performances. Watch the episode with Geri Halliwell, where she appears to basically be trying to copy the Samantha act, and it's one of the most horrifying things I've ever seen in my life. Cattrall was effortless, it was a HUGE performance, and yet never for a second was there a trace of phoniness in it. You could easily be forgiven for thinking she was just playing herself. It has that Stallone in Rocky type quality where you think that's just them, then you see them off camera, realize they're nothing at all like that, and are sort of in awe.