From my experience.
Microgaming is the hands down winner for highest player value. Some of my historically top performing casinos have been captain cooks, spin palace, music hall, colosseum casino.
Playtech seems to convert players a lot better than microgaming but the player value and player retention are not quite as high. Golden Palace is my pick out of this bunch.
I also like Slot Land and party poker for poker.
The list of who I dislike would be a lot bigger.