Top picks in no particular order:
- Sea Beggar (Best naval UU IMO, even better than the great Ship of the Line due to its combination of free promotions, unit capturing, higher CS, and earlier tech unlocking. Allows the Netherlands to take control of the sea at a really important period of the game, and a smart human player can leverage that to control the oceans for the rest of the game.)
- Samurai (They are not that great on their own but their double XP and GG generation promotions tie perfectly into Japan's UA, and combined with the Dojo promotion they can rival just about any other melee UU throughout the rest of the game, ASSUMING you can keep them alive and highly leveled.)
- Impi (Still some of the most terrifying offensive infantry in the game, especially combined with their special UB promotions)
- Slingers (They sit on mountains and pelt the enemy to death with stones, and can run away when charged. One of the most defensive early-game units, and their promotions stick with them through the ages giving the Inca extra-great ranged defense units all through the game.)
- Mandekalu Calvary (Again, not that great on their own, but prior to the Spearman/Pikeman upgrades with the last patch these were the absolute best at early-game city-sacking rushes. Combined with their UA and special promotions they are lightning fast and can generate a lot of early gold that feeds back into your economy when you're trying to rush down neighbor's capitals in the early game. The "Scissors" to the Slinger's "Paper".)
- Pictish Warrior (Competes closely with Hoplites and Immortals, but I give best pick to Pictish due to the Faith on Kills ability which allows the Celts to consistently power their way into an early religion. Controlling a religion is still very powerful, enough to make Pictish a top pick even though their Faith generation goes away once they upgrade to Pikemen.)
Honorable Mentions:
- Caroleans, Musketeers, Legion, and Ships of the Line (for all being solid choices that can be the best units of their era unless certain other Civs are in play that outclass them)
- Nau, Merchant of Venice, and Siege Tower (for their utility, which is difficult to compare to pure military units)