Ranked in terms of being most popular:

1. [[Yuriko, the Tiger's Shadow]]. Ninjas are a fairly popular tribe and had been lacking a proper general for a long time now. Additionally, she plays very nicely with the slew of Dimir colored skulk / unblockable creatures. Plus, 'Commander Ninjitsu' is pretty nutty, as bypassing commander tax is always a strong effect.
2. [[Varina, Lich Queen]]. Zombies is the most popular tribe in EDH (at least, the last time I checked.) However, Varina finally gives players the option of Esper zombies and white token producers / doublers. She also has a looting effect as well as token production attatched to her. She seems like a pretty damn strong commander for the zombie tribe, and I think people know it too.
3. I really can't decide between these two, so we'll call it a tie for 3rd between [[Tawnos, Urza's Apprentice]] and [[Tuvasa the Sunlit]]. Both fly low to the ground with 2 and 3 CMC respectively, but Tuvasa is probably stronger on her own without dedicated synergy pieces and offers players another great enchantress commander with some really nice effects. Tawnos offers some spicy plays alongside powerful artifacts and creatures. Both seem strong and entertaining to play, hence my inability to choose one over the other.