As far as Paramount is concerned the tv series alone are canon. The book don't have a set continunity and at times contradict each other. Then you have the online and video games that contradict each other as well. Star Wars was the universe that George Lucas exorcised aboslute creative control over to keep everything organized.
As far Star Trek:
Tv Series: The Q are the most powerful beings around. Invincible to anything except another Q. Nothing we have seen contratics this. Other beings may be powerful, but not have demonstrated to being close to the level of the Q.
The Novels are more complicated because that contradict each other. I haven't read the novels, but from what I gather from reading articles online...
The Creator of the Star Trek Multiverse: Either The Lady or THEM. The entire continuum is powerless against them.
Second Level: Q Continuum/M Continuum - The M were introduced as some sort of counterbalance to the Q and are just as powerful.
Third Level - Zero - He is more powerful than individual Q, but not by much and certainly not the entire Continuum.
Fourth - Individual Q
Fifth - Organians, I once read an except from the battle between the Q and Zero. The Organians observed and Q commented that while they were highly evolved compared to humans they are still a good ways below the Q.
The rest is speculative since omnioptent beings rarely confront each other and it is impossible to measure without a direct confrontation.
Also the Douwd are in there.