The reason why he is and probably always will be DC’s best cosmic threat character is because he is in fact a “character” and not a plot device. Most big cosmic threats (The Anti-Monitor for example) were created for events or crossovers, they have very little in the way of personality and are pretty much just a cool design and vague power set in search of a personality( some eventually do find a personality but it’s usually well after the fact ) Darkseid on the other hand really benefits from being the primary villain in the whole Fourth World mythology first and foremost. It gives him an actual personality, backstory, family history etc. So when he combats the heroes of the DC he isn’t some amorphous evil power blob but is in fact a character.