Who were genetically superior, the Founders or Species 8472?

[Distorted Humor]

Uh, how do you rate "genetic superiority" and by which criteria?

As far as we know, Betazoids and Denobulans never started a war or committed an act of genocide. If we assume that their genetic makeup makes them less prone to aggressive behavior, I'd argue that those species are "genetically superior" to most others in the Star Trek unverse. Especially Species 8472.

isn't aggression something that's considered a huge positive?

That...depends...

Spoiler: Ringworld by larry Neven Spoilers

In the classic Novel "Ringworld" by Neven (Ringworld created the idea that now known as "Halos" from the game Halo...), it is revealed that the Puppeteers lured the outsiders to a human colony so Humans could purchase from them FTL drive. With FTL the humans where able to defeat the very aggressive Kzinti species, who where too aggressive (for example, they launched wars with humans always before they where ready.) after a number of wars (I think five) with 1/3 of the most aggressive Kzinti killed, the species had gone from a race that just wants to kill and eat other races to one that can co-exist with other species.

 

[Anwar]

The Kzinti's problem wasn't aggression, it was their near-total inability to think before acting. And by the time they had learned to do that it was too late.