Knowing that, the best faction combination not taking alignment into account would probably be Tyranids or Necrons.
Agreed; Tyranids could quickly get ridiculous if you gave them access to some of the low-cost command units in other factions. Their major problem has always been that their cards are high power/utility, but with accompanying high cost and a serious difficult in being able to pull the right card at the right time (ever played an infestation deck, but not drawn any infestation? Yeah).
If you could bulk up their command presence with a few decent 1-for-1 cards, though, you'd be able to actually field a swarm, which could quickly get ridiculous, particularly when paired with their Synapse unit.