It's difficult to say that one of the casino software is the best, but still, there are 3 world leader and they are: Microgaming, Playtech and Real Time Gaming. Personally I love mostly casinos with either Microgaming or Playtech software. I like to play video slots and I found that the best videoslots are in Microgaming Casinos.
There are many other software for Casinos, all listed here are licenced and ok. Just avoid playing in casinos with own software - may be it's ok, but maybe not...
