It really depends on what you are after. If you are looking for the best casino as an affiliate, it is the one that converts best for you and brings in the highest return. As a player there are a bunch of factors like games, promotions, and even the people that play there depending on the games you play.
I don’t think I could ever recommend just one because each has its own unique flavor. Its like beer, I have some that I prefer, but heck, I’m not going to turn down a beer if its offered.