Well i have two favorites, Songhai and Vetruvian, but honestly i think i like Vetruvian a little bit more.
I like both of those factions because they have very different playstyles than the other factions and they presented things that i never saw before.
Vetruvian is extremely unique with their mechanics, especially the “Summon Dervish” from the obelysks, which can be a real challenge to use sometimes. I also like how they can punish their opponent by their own positioning via blast and star’s fury. Those are things that i never saw before and they explore a lot the board. IMO Vet is one of the least straightforward factions, so you can make lots of different decks.
Songhai is my second one mostly because of their positioning interaction and different playstyles.
Also both factions are the most “Fire-Themed” factions and that’s a plus for me ;).