I can personally recommend Casino Extreme as I have played there on and off for a few years and they also have instant cash-outs.
The same people running Casino Extreme also run Casino Brango which uses the NuWorks which is very similar to RTG with slightly different games.
Both of the above we have free monthly private slots tourneys with cash prizes (you can find these in the Slots Tourneys / Private Slots Tourneys forums).
I hope this helps and perhaps others also have different recommendations.