Hard pick between Psynumbra, Voidtech and Xenoplague.
Psynumbra cus evil shadow demons and madness, Xenoplague cus evil virus and Zerg like mutants and finally Voidtech.
I've picked Voidetech cus I think it's cumulatively the most useful ST of all. Early unit that spams clones, so many useful tactical/strategic operations (teleport, clone unit, economy boost doctrine, revive unit, slow approaching enemy stacks and boost defense of your colonies). Plus their units are all generally very useful. And it's as sci-fi as it can get and probably is the one sitting in the strongest with the lore of the game.
Promethean is cool but somehow I enjoy it a bit less then the three above, even though I'm perfectly aware that it is a very strong ST. It's too straightforward for me, everything is about doing more damage/status effects.
Actually I enjoy all ST, probably the only reason I would rate some of them a bit less is that I didn't play them that much (Celestian for example).
And unfortunately I must agree that Heritor isn't exactly as good as the rest. I mean the ST is fine and unique but the Essence mechanic can be a bit tedious to use effectivly. The game already has many mechanics to take into account so adding another layer on top of the existing ones adds to the complexity. And yeah it feels too much aloof from the rest, but guess that's what you get with an OPTIONAL dlc content that might or might not be used in the game. So in this sense I'm ok with it.