If I had to say one, it'd be necrophages, both as a normal empire and as a hive mind. Very fun super-aggressive, with a unique balance of bloom or burn. There's just so much versatility in it, and it keeps me from a default run of xenophile-egalitarian. As a hive it taught me more about early-game military rushing than anything else, and as a non-hive it taught me about the pitfalls of bloom-building when taken to its extreme.
I've also come to really enjoy Angler-Catalytic trade builds, and when doing that there's really no better start than Lost Colony. Aside from the capital modifiers themselves, I'm convinced Lost Colony's real strength is that it ensures more of your empire population across the galaxy, letting you fall in on your pre-defined master race template rather than dealing with AI RNG.
When I'm machine, it's usually Rogue Servitor, which is either Remnant or Shattered Ring. Remnant is strictly better, probabably, but it's nice being able to get a Ringworld up before the game is over.
Teacher's of the Shroud is pretty swanky, and is becoming my go-to for spiritualist for obvious reasons. Really want to do it as a Barbaric Despoiler some time in the future, to get the effect of 2 more ascension perks than usual. Just wish I could combine it with Necrophage.