plasmacannon
Emperor
Does the eagle warrior work against barbs? If so then it seems clear and away the best one. Consider how important stealing workers was on the higher difficulties in Civ5 AND that you're going to need a steady stream of builders in civ6, this is going to be a strong one.
Even if the conversion doesn't work against barbarians but only against major civs and city states, you could easily turn a neighbour into a builder farm.
No the Eagle Warrior special ability to gain builders does not work against Barbarians.
Aztec look for early on. It depends on how quickly a civ can tech. The intro videos weren't exactly what a great player's strategy would be. They were more about showing is the different things in the game.
Saka Horse Archer looked good at first, because of it's double production rate, but at 100 production, it basically costs the same as two units, so that isn't its big advantage. It is speed 4, which is very nice to outmaneuver an opponent and pillage districts. It is still weaker than Egypt's Maryanu Chariot Archer which is speed 4 also.
As someone else said, based on when I usually fight, I would want a unit from that era.
I don't know it's strength yet, but I'm going with the Redcoat. It will have to be stronger than a Musketman and it gets a bonus fighting on other continents, which should be most of the game.
It gets no penalty for disembarking, so opponent units near the shore could be attacked more easily.
It's Sea Dog ship can Capture ships, so it England will be building up it's navy rather quickly.