Do your research and make a list of 10 based on information on this site and other resources like gpwa. (Based on where your traffic comes from, the languages that you need and the feedback on the program on customer service, payment etc)
Then halve that number and go with 5 initially . Make sure you have decent spread of different software platforms as well- as this should increase your overall conversion, say go with 2 microgaming, 2 playtech and a cryptologic.
At the end of the day, the only sure way of finding out is to test yourself, so periodically throw another one into the mix. Try and work out why your best partner is working so well, and try and replicate.