Everyone has their favorites, and it also depends on the type of visitors you have.
They all exist because different things appeal to different people.
I will try to give a variety of softwares:
Vegas Affiliates, Wager Profits, Casino Blasters, Income Accesss, Bingo Rewards, Casino Rewards, Poker.com, Paradise Poker, Poker Room, Gaming Partners, Jackpot Palace, Metro Casino, Refer Bingo, Slotland, Share the Rake, Bingo Royalty, Play United, Referback, Focal Click Casinos, Trident Share.
I like these and others. Everyone has their favorites. The ones I listed have been fair as far as I can tell. They are all responsive and supportive. I picked a variety of softwares and included Casinos, Bingos and Poker.