I find that Microgaming has the widest selection and best performing slots. Personally when I play slots online I generally go to a MGS casino.
Playtech is also a solid platform for slots.
For Video Poker I like RTG or Playtech.
For table games I like Playtech or Cryptologic.
Please note: These selections were based on personal play preference. In the case of slots MGS is also my highest earner for that segment.