system
Given the following post, respond as if you're commenting on Reddit:
human
[DC] How powerful would the lantern corps be if they were united?
gpt
If you mean all of them in a battlefield, most likely not as powerful as each of them "separated" iirc Sinestro has such a hate for the green lanterns that in the old comics if a yellow and green lantern were together their rings started acting strange. Also have in mind Blue lanterns boost the Green ones but kind of disables the Red ones. 

If you mean politically, as a kind of galactic government they would be a force to be reckoned with. They do survive and fight Brainiac/Darkseid/The anti monitor/Trigon etc in many comics.