If most of your traffic is from the USA, then online casinos that accept U.S traffic should convert well for you. Mainstreet affiliates are an excellent group to promote to U.S traffic.
If on the other hand most of your traffic is foreign, then I’d recommend europartners. They’ve always performed very well for me.
You might also want to check out the “Commissions Received” section on CAP. It will give you an idea of which sponsors are converting really well.
Another thing I do is to look through the big affiliate sites, just to see who they are promoting. If a group of high profile affiliate sites are promoting the same sponsors, then it usually means that these sponsors are worth promoting. High profile, busy sites that spend lots of $$$ on advertising aren’t going to promote crap casinos.