Top Tier:
Keshiks - ranged mobility ftw, extra great general generation pairs nicely with Mongolia's other UU.
Samurai - tough unit that produces extra great generals is always nice, combined swith japanese ability this unit can be used for kamikaze attacks even when vastly out tech'd
Chu Ko Nu - double the attacks virtually doubles the damage your army can do
Narusan's elephant - much stronger than anything else in it's era, huge bonus vs mounted (the unit it replaces is the mounted knight). doesn't require horses unlike the knight. Can even be used extremely well against renaissance era mounted units.
Longbowman - incredible range, what else can i say?
Ship of the Line - only naval UU makes this a great UU - your rule of the sea will be uncontested, i would put this lower due to pangaea maps, but who the hell picks England and then picks pangaea?
Janissary - healing when destroying another unit is incredibly good, allows you to save promotions for really important things, Riflemen upgraded from these are great
Companion Cavalry - Yup, still got it (even since the horseman/mounted nerf)
Good tier:
Hoplites - bloody strong, straightforward unit
Legion - road building and extra strength for the win
Babylonian Bowman - great early boost to an otherwise non-military civ, bowmen being defense focused helps
Landschnekt - supremely spammable.
Mowhawk Warrior - When this unit arrives there is still usually a lot of forest on the map especially given the Iroquois start bias, complements them nicely
Khan - great synergy with the Keshik, these two make Mongolia a medieval era power civ. The fact it's not a fighting unit is a strike against it.
Tercio - stronger Musketmen with the bonuses of pikemen and none of the inherent weaknesses of musketmen (to cavalry). What's not to like? Not sure if they keep the +100% vs mounted when upgraded, if not they may get moved down a bit.
Jaguar Warrior - Great bonuses in jungle (aztec start bias) and when killing units which carry on when upgraded, making this early unit worth making despite its early entry. Plus you get a free one at the start of the game.
Mandekalu Cavalry - The only mounted unit with an actual bonus vs cities, combined with the Songhai ability this is a good combination (MOAR gold), these guys actually do pretty well vs cities even walled ones.
B17 - Bombers become pivotal if you have air control late game, and the B17 is very good at what it does.
Mid tier:
Panzer - In itself a very good unit but Modern Infantry are just so much better to go for - tech tree needs rearranging to make these (and tanks) more useful. It doesn't make sense that you can get Mechanized Infantry without first researching Tanks and Infantry.
Maori Warrior - I guess these guys have yet to prove themselves, they also come very early before much warfare happens, their haka bonus is kept when leveling up though which is cool and you get a free one at the start of the game.
Ballista - A good unit to back up your legions, but weak to cavalry and it competes with your Legions for precious iron.
Cossack - Virtually the same as the Cavalry but with a nice bonus on flat ground. Comes a little late (mounted units will soon be mostly obsolete by the advent of artillery and armor). At the very least Russia's good ability ensures you won't run out of horses for these guys and Russia's Krepost means he will probably have a free promotion. This unit could do with a little boost (nothing huge).
Crap tier:
Slingers - a very meh unit, at least the Incas have other great abilities to make up for this unit. I realise the devs were probably struggling here for a historical UU
Zero - Not a completely horrible unit, but it comes way too late and has a bonus vs helicopters and other fighters when it would be better (though less historical) to have a bonus vs bombers. Japan's awesome ability (top 5 in the game) and other UU which is also very good make up for this luckily. Another idea would be to give it a 100% bonus vs naval units to simulate Kamikaze attacks.
War Elephant - Probably the most useful of all these, bordering mid tier. This unit's strength makes no sense compared to Narusan's elephant which is a beast. Horsemen come soon after. Not a whole lot of people build chariots anyway, at least this guys doesn't cost you any horses. On the plus side it's a lot more durable than the horse archer and doesnt have the weakness of the wheeled unit.
War Chariot - similar deal to above. except it costs you horses which could be better used on Horsemen.
<<As yet unrated>>
Minuteman
Foreign Legion
Musketeer
Camel Archer
Immortal
Sipahi
Conquistador
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Here is an updated list. I'm particularly not sure about War Chariots, War Elephants.
Any suggestions/dissagreements guys? The newer DLC units (particularly spain's) are a bit unproven. How do you guys find them?