Yes, you last two finally mentioned it.
Celts Gallic Warriors with G3, but, you forgot they can get W3 too, so, it has 2 first strikes and it heals.
They are the only unit before gunpowder that can nulify all terrain bonuses of the defender except fort.
Stick a Great general on it and you have Leadership 100% xp gain (great for Boudica's Charasmatic trait) and Tactics 30% withdrawl for a total of for 80% withdrawl after G3's 50% is added in. As you keep fighting you give it Combat6 for the 25% bonus.
Great early unit.
Cho-Ku-Nu as mentioned earlier. The only non-seige unit in the game to cause collateral damage. Give him G3 and he gains withdrawl 50%, to keep earning more xp.
Give all of the Combat and Drill Promotions, for a strong early machine gunner type unit with 5-8 first strikes. Nice.
Similar strategy for the Samurai (starts with 2 first strikes and Drill 1).
All Combat and Drill promotions and Woods 3.
The only unit before gunpowder that can get woods 3 and Drill 4, for a total of 7-10 first strikes!
Slice and Dice.
Honorable mention to the humble scout.
With your early scout, give him combat 1,2,3, so, you can give him Sentry.
Upgrade him to an Explorer, which gains G1, W1 free, then, further upgrades goto W2 and G2.
If he is still alive, Medic 1, Combat 4, Combat 5, and Medic 2 to make him your stack's healer.
You probably need a Great General for all of that, but, who would want to waste him here?
Only if, you have the Great Wall, you are Cyrus and you have GGs to spare.
I've had 7 GGs in one game before finishing it, and I know that has to be low for the hard cores on here.
The obvious:
Numidian Calvary Conbat1 and Shock lets them take out the spearmen sent after it. More Combat is even better. Flanking 1+2 help it survive, and Charge for a higher base vs catapults.
Impi with W2 can move 4 tiles!
Always upgrade your Quechas to combat 2 before upgrading them to axemen, to keep their free combat 1 promotion. If you go Woods 2 and forget the combat 2 and upgrade it to axemen, you will lose the Combat 1.
Immortals with Combat5 and shock capture lots of cities.
Flanking 1+2, when, you get a chance later.
For everyone that attacks cities. Best Trebuechets are from Spain. Check Unrestricted leaders and then choose Churchill as your leader. He is Charismatic/Protective.
Cha effects all units including Trebs. Protective allows you to build your walls and Citadels quickly.
Barracks 3 xp, Citadels grant +5xp = 8xp or Trebs with City Attack 3 out the door. If you have Vassalage +2xp, and Theocracy +2xp and a GG +2xp for a total of 14xp easily achieved every game to destroy cities. Spend the extra xp towards Barrage 1,2,3 for more damage on those units.
I have found giving Shock to a spearman vs the AI, lets me actually attack the mounted in a stack.
Janissary's with Combat5 and cover to deal with pesky Longbowmen. Probably the best unit to place a GG on (or a tie with Gallic warrior and discusting Combat 5, City Attack 3 Preatorian)
Spec some out to have woods3 (to heal),
City3 to defend cities,
Hills3 to protect Iron + Gold mines.
Caravels with Flanking 1+2, to reach Nav 1+2, to more quickly discover the world is round, for an extra +1 movement. Makes destroyers sickingly fast.
Privateer get Flanking2, Nav2, Combat5
Skirmishers with City3 are almost a cheap Longbowman. Especially, with G2 on a Hill defending your city. and Combat 5 (if you can get it).