Voted for Starro, though now that folks point it out I suppose it's not an origin story at all. And I've never read 144 but it sounds real damn interesting. Well, between Appelexians and Darkseid, I'd pick the element aliens over the Fourth World. That's not a fight the heroes should engage in so early in their careers; Darkseid is something you build up to over time.
Picked Starro largely just for the concept. It's not a 'supervillain' so much as a natural disaster, like locusts. Requires a different kind of response than a typical space invasion by Mongul or somebody. Got a great "Aliens meets zombies" vibe. And you can get some great hero v hero fights out of it as people get fish faced.