The answer to that will vary from webmaster to webmaster.
I find that, based entirely on conversion (as percentage of traffic sent), these guys perform well:
– adriches – phoenician casino
– Wager Profits – Sci-Fi Casino group,
– Fortune Affiliates casinos
– Winward casino (I’m still new with them, but very good conversions for the amt of traffic I’ve sent)
I make good money with other groups, but these guys have the best conversion rate for me, even if I don’t make the most money with them, I expect the payouts will only get better.
They also have great affiliate relations, which is important.
A lot of webmasters love Slotland, too. I’m just giving them a real push this month, so I can’t say from personal experience. Never heard anything bad about them, but a lot of good.