Dependant on to many vairbles.
The traffic you supply
The payout structure you are on (CPA, Rev share)
A very broad question.
As an affilate it is important to define YOUR goals then find a group that matches those goals. i.e If you are after a set number of new depositors a month then you are looking for a group that will convert/induce that first time deposit. If on the other hand you are on a rev-share you are looking for a group that has a strong hold on players.
Define what it is you want – find the group that matches.
There are no keys/perfect answers to this – and no shortcut’s I’m afraid