What do you want from the library? If you want "Hollywood" sounding FX tracks then it's not about the library it's about the editing.
That said Sound Dogs has most of the "professional" libraries on line. But getting decent on line SFX can get expensive fast. So again it comes down to what you want from the library. Are you building a personal library because you want to do sound or are you looking for a few specifics for a project or maybe just some general sounds to have around? For the middle one Online is probably a good choice for the first and third not so great. If your building a library I would start with some really wide stuff like Hollywood edges Super Singles, or the BBC collection. Digital Juice also has a big collection but not as well documented or easy to use as the others.