Personally:
Least: Warhammer and its Universe, hopeless and chaotic. Even the golden age of humanity is built by war and conquest. And even the pique of technological development has stagnated. The world in general points to the forces of chaos eventually overpowering humanity. And the other alternative races are not appealing either. Even the Tau are naive and might succumb to chaos eventually.
Most: Pretty much any omegaverse: the idea of a soulmate who’s destined for you fills the hopeless romantic in me. I like the idea of nesting, skinship and even dominance and submission (wholesome mind you), and scenting. Even if at first glance there may be disparity between the sexes and ABO classes, their equal roles in societies can be evened out, depending on what canon you applied to it. On the flip side, you can also power trip and be born as an Enigma, a rare form of ABO who can dominate even alphas. The possibilities are endless. :)
How about you?