So I'm going to give each unit in CoW a Letter grade of A, B, C, D or F, where A is for the best units and F is for the worst.
Note: these are OFC biased toward me and my playing style.
If you disagree with me, that's fine, but don't attack me because of it.
We'll go in order of the categories:
Unarmoured
Militia - C
Yes, I know lots of people here use Militia as a prime ground unit, but I prefer to use Infantry for that purpose. I only research the first level of Militia, and usually use them for holding cities and keeping them from rebelling.
Infantry - B
A great defensive unit, I keep these one the frontlines as well as the homeland. If I happen to get attacked during the night, they prove very useful.
Mot. Infantry - A
I've done experimenting on which Unarmored unit (Mot. inf, Mech. inf, Commandos or Paratroopers) are the best. The verdict is mot. inf give you the most bang for your buck. They're the least expensive and unlock the quickest, and are also the fastest unarmored unit. Bonus: they're also Scouts.
Mech. Infantry - B
This unit has identical stats when attacking and defending, so it's not a horrible unit. However they can be hard to afford in the early game and unlock late(r).
Commandos - C
More expensive then mot. or mech inf, and usually unlock later than both. Only reason I would make them is if I'm playing a mostly mountainous country like Tibet or Switzerland.
Paratroopers - C
I don't really think the Paratrooper landing strategy is very useful, whenever I build them I always keep them in ground form. They're actually pretty good combatwise (10 attack vs unarmoured) but are expensive and unlock later.
Ordnance
Anti-Tank - D
Only need these if you don't use Artillery or run into a guy who has a TON of armour units.
Artillery - A
Excellent unit, allows you to engage enemy stacks without taking damage. Works especially well against armour, but once in higher levels it's good against infantry as well. Just make sure to keep them out of close combat, as they are crap at defending.
SP Arty - C
Just like arty, but (slightly) better statwise, and more expensive. I generally don't use them.
Anti-Air - B
Cheap unit that provides stacks with suitable AA defense, as well as significant heavy armour defense at higher levels. Only drawback is they're one of the slower units in the game, so it can be a drag for players who like fast attacks.
SP Anti-Air - D
Faster than regular AA, but more expensive and unlocks later. Hardly any difference statwise.
Tanks
Armoured Car - B
Great defensive unit, great for countering units like mot. inf. Best to keep grouped with other armour units, as it's only effective at killing unarmoured stuff.
Light Tank - A
Overall a good unit, pretty cost efficient, fast, good for accompanying Armoured Cars. I usually throw a couple into my main attacking stacks.
Medium Tank - F
Expensive, slow, only good at killing Light Armour. Very vulnerable to air and artillery.
Heavy Tank - F
These units can be good if used right (and with fighting inactives, as @jubjub bird has showed us), but I don't use them in public games. They're not as good at attacking heavy armour as you would expect, and are VERY slow, and are VERY vulnerable to air attacks, artillery, and tank destroyers.
Tank Destroyer - D
Good for countering MT and HT, but not much else. Definitely not worth researching later levels.
Air
Interceptor - A
A must have for any player, regardless of what other air units you produce. Air superiority is key to winning.
Tactical Bomber - A
Great for attacking unarmoured and even light armour, more HP than attack bombers.
Attack Bomber - D
Don't really need them if you have arty. Low HP and expensive.
Strategic Bomber - F
I don't want to destroy the enemy's level 5 Naval Base and Industry factory (why destroy buildings when you can use them yourself?) Expensive and can be shot down easily.
Naval Bomber - B
Great for reacting to surprise naval attacks, as well as attacking unescorted convoys.
Naval
Destroyer - B
Fastest naval unit, scout, and kills subs. Only bad thing is it takes lots of damage when killing subs.
Submarine - B
Good for scouting opponents coastline and for sneaking up on lone Battleships. Can be easily defeated by Destroyers or NB though.
Cruiser - A
Provides needed AA support for naval stacks.
Battleship - A
The most powerful naval unit. Get enough of these and almost nobody can stop you.
Aircraft Carrier - C
Usually not needed, can be a "toy" to play with in the late game. Expensive and vulnerable.
Secret
Rocket Artillery - C
I don't really use them as I usually make mot. inf + tacs, and my RMs are better spent elsewhere.
SPRA - B
Rocket arty on steroids, another toy to play with in the late game.
RRG - A
Largest range of any unit, great for taking out Airfields as well as armoured units. Vulnerable to air attacks though.
Flying Bomb - F
Worthless, low damage and easily shot down by AA.
Rocket - D
Better than FB, but still mostly worthless. Great for destroying enemy airfields, but not much else benefitting them.
Rocket Fighter - D
Small range, can't land on carriers... not needed if you have enough ints.
Nuclear Bomber - A
Can take out an entire stack of units with one attack. Very expensive, but can make (noob-y) enemies afraid of you.
Nuclear Rocket - C
More expensive than nuclear bomber but only slightly better statwise, usually the game will end before it can even be used.
Note: these are OFC biased toward me and my playing style.
If you disagree with me, that's fine, but don't attack me because of it.
We'll go in order of the categories:
Unarmoured
Militia - C
Yes, I know lots of people here use Militia as a prime ground unit, but I prefer to use Infantry for that purpose. I only research the first level of Militia, and usually use them for holding cities and keeping them from rebelling.
Infantry - B
A great defensive unit, I keep these one the frontlines as well as the homeland. If I happen to get attacked during the night, they prove very useful.
Mot. Infantry - A
I've done experimenting on which Unarmored unit (Mot. inf, Mech. inf, Commandos or Paratroopers) are the best. The verdict is mot. inf give you the most bang for your buck. They're the least expensive and unlock the quickest, and are also the fastest unarmored unit. Bonus: they're also Scouts.
Mech. Infantry - B
This unit has identical stats when attacking and defending, so it's not a horrible unit. However they can be hard to afford in the early game and unlock late(r).
Commandos - C
More expensive then mot. or mech inf, and usually unlock later than both. Only reason I would make them is if I'm playing a mostly mountainous country like Tibet or Switzerland.
Paratroopers - C
I don't really think the Paratrooper landing strategy is very useful, whenever I build them I always keep them in ground form. They're actually pretty good combatwise (10 attack vs unarmoured) but are expensive and unlock later.
Ordnance
Anti-Tank - D
Only need these if you don't use Artillery or run into a guy who has a TON of armour units.
Artillery - A
Excellent unit, allows you to engage enemy stacks without taking damage. Works especially well against armour, but once in higher levels it's good against infantry as well. Just make sure to keep them out of close combat, as they are crap at defending.
SP Arty - C
Just like arty, but (slightly) better statwise, and more expensive. I generally don't use them.
Anti-Air - B
Cheap unit that provides stacks with suitable AA defense, as well as significant heavy armour defense at higher levels. Only drawback is they're one of the slower units in the game, so it can be a drag for players who like fast attacks.
SP Anti-Air - D
Faster than regular AA, but more expensive and unlocks later. Hardly any difference statwise.
Tanks
Armoured Car - B
Great defensive unit, great for countering units like mot. inf. Best to keep grouped with other armour units, as it's only effective at killing unarmoured stuff.
Light Tank - A
Overall a good unit, pretty cost efficient, fast, good for accompanying Armoured Cars. I usually throw a couple into my main attacking stacks.
Medium Tank - F
Expensive, slow, only good at killing Light Armour. Very vulnerable to air and artillery.
Heavy Tank - F
These units can be good if used right (and with fighting inactives, as @jubjub bird has showed us), but I don't use them in public games. They're not as good at attacking heavy armour as you would expect, and are VERY slow, and are VERY vulnerable to air attacks, artillery, and tank destroyers.
Tank Destroyer - D
Good for countering MT and HT, but not much else. Definitely not worth researching later levels.
Air
Interceptor - A
A must have for any player, regardless of what other air units you produce. Air superiority is key to winning.
Tactical Bomber - A
Great for attacking unarmoured and even light armour, more HP than attack bombers.
Attack Bomber - D
Don't really need them if you have arty. Low HP and expensive.
Strategic Bomber - F
I don't want to destroy the enemy's level 5 Naval Base and Industry factory (why destroy buildings when you can use them yourself?) Expensive and can be shot down easily.
Naval Bomber - B
Great for reacting to surprise naval attacks, as well as attacking unescorted convoys.
Naval
Destroyer - B
Fastest naval unit, scout, and kills subs. Only bad thing is it takes lots of damage when killing subs.
Submarine - B
Good for scouting opponents coastline and for sneaking up on lone Battleships. Can be easily defeated by Destroyers or NB though.
Cruiser - A
Provides needed AA support for naval stacks.
Battleship - A
The most powerful naval unit. Get enough of these and almost nobody can stop you.
Aircraft Carrier - C
Usually not needed, can be a "toy" to play with in the late game. Expensive and vulnerable.
Secret
Rocket Artillery - C
I don't really use them as I usually make mot. inf + tacs, and my RMs are better spent elsewhere.
SPRA - B
Rocket arty on steroids, another toy to play with in the late game.
RRG - A
Largest range of any unit, great for taking out Airfields as well as armoured units. Vulnerable to air attacks though.
Flying Bomb - F
Worthless, low damage and easily shot down by AA.
Rocket - D
Better than FB, but still mostly worthless. Great for destroying enemy airfields, but not much else benefitting them.
Rocket Fighter - D
Small range, can't land on carriers... not needed if you have enough ints.
Nuclear Bomber - A
Can take out an entire stack of units with one attack. Very expensive, but can make (noob-y) enemies afraid of you.
Nuclear Rocket - C
More expensive than nuclear bomber but only slightly better statwise, usually the game will end before it can even be used.
“A battle fought without determination is a battle lost.” - Josip Broz Tito