I am also looking for other sites best used for trading NFTs, if you know any good marketplace too, let me know.
OpenSea is the most popular for the time being:
https://opensea.io/There are more that offer NFT trading like Enjin Marketplace and Rarible which you mentioned.
Also WAX:
https://wax.simplemarket.io/I've only traded on OpenSea and stopped after offloading some NFTs I was holding. I have no experience on the rest.
Yes, I would say that opensea.io is the most popular right now. One good indication? cyber criminals are trying to create a fake site already. So it only means one thing, it's the go to for NFTs right now.
As far as the sites you mentioned, I haven't heard about it, so the rule of thumb is to do your own investigation before you post your art. Good luck!!