There is no doubt that Trigon is a huge threat to the DCU. In a sense, the DCU has never quite experienced the full consequence of Trigon's campaigns in this dimension.
However, I think there are entities in this list who are more of a threat, and it seems most of the stories involved with Trigon have been with the Teen Titans. You would think that such an all-powerful entity would require the full focus of the JLA not only to fight him but to make sure that he never comes to this dimension.
Once again, he's really bad, but probably not the worst compared to the Anti-Monitor and even the natural force of entropy in the form of Imperiex Prime.
Consider controlling reality. Most of the people on the list can do that, even Mxyzptlk. Immortality? Evil doppelgangers? Planet destruction? Almost everyone on this list can do that.
Don't get me wrong--he is a threat, but there are worse.