Training Camp:

Knight(D): Outclassed by Valkyrie, Mini PEKKA, Lumberjack and Mega Minion, to name some. Hardly seen in Arenas 4+.

Arrows(D): Outclassed by many other spells. Cannot kill troops more than Minions, and not fast enough deploy time. By the time it reaches the Skarmy your tank's already dead. Rarely seen in Arenas 5+.

Archers(B): Cannot be killed by spells for elixir advantage. Counters most cards well and targets air.

Bomber(C): Deals high damage and counters swarms well, 360 degrees splash. Cannot target air, however. Loses usage at Arenas 6+.

Giant(B): Cheap for a burly unit, tanks well. When paired properly, it is nearly unstoppable. Mostly stopped by Inferno Tower.

Fireball(B): Good for making positive elixir trades and killing support units. 

Mini PEKKA(C): Huge damage but single target and cannot hit air, slow attack speed. But in hands of a skilled player, devastating. Kills a lot of stuff.

Musketeer(B): Very useful. Relatively high damage, targets air, hits quickly and long range. 

Baby Dragon(F): Trash that doesn't kill Goblins in one shot. And it's an epic.

Witch(C): Good support unit but killed by Valkyrie, Fireball, Rocket and Lightning. 

Prince(C): High damage, especially when charging. Easily distracted and killed. In hands of a skilled player, devastating.

Skeleton Army(B): Great for whoever doesn't push with splash(aka nobody), but great all the same. Gets destroyed by Zap, the Log and stuff.

Goblin Stadium:

Goblins(C): High damage but low health. Decent destraction but not used a lot.

Spear Goblins(E): Walking trash. Low damage and low health. Range and attacking air don't do much.

Valkyrie(B): High health and moderate 360 degree splash damage. Very useful but low speed and inability to attack air.

Goblin Hut(F): A hut that spawns trash. Really?

Lightning(C): Situation-dependent. Useful for striking down multiple glass cannons but relatively high-cost.

Goblin Barrel(C): Also situation-dependent. If opponent doesn't have zap or log, does lots of damage.

Bone Pit:

Skeletons(D): Nerf in count has robbed it of its usage. 3 skeletons is a much less decent destraction, but still can do high damage.

Minions(B): Flying horrors. High damage and immune to Zap or Log. Attacks quickly and only 3 elixir.

Tomstone(B): Good distraction and Zap bait. Shuts down Sparky and Inferno Dragon.

Giant Skeleton(E): Only good if your opponent has no good cards to counter, and even if bomb does reach the tower it still does not guarantee a win, and high cost.

Balloon(C): Handy for extra damage to tower, but only worth it if it gets at least one hit on tower. Destroyed by Inferno Tower, Minions and Mega Minion.

Barbarian Bowl:

Barbarians(D): Ever since the buff in damage of PEKKA and Prince, usage has dropped. Still handy for some cards like Hog Rider. Also weak to Fireball/Rocket and costs 5 elixir. Also moves slowly.

Cannon(C): Good distraction building but low health and high internal damage(damage it receives every second, passive).

Barbarian Hut(F): Spawns 2 Barbarians which do nothing. A simple counter would be Rocket or Furnace.

Rocket(C): Situation-dependent. If opponent uses glass cannons or buildings, it is very useful for killing them and also damaging the tower.

Rage(D): Rather useless but only 2 elixir, and good for extra chip damage.

X-bow(F): I'd say every deck has a counter to this trash.

PEKKA's Playhouse

Minion Horde(C): Useful for the mid-Arenas but will soon become increasingly unreliable. Kills a lot of stuff as well, but Minions would do fine.

Tesla(C): A cannon with more health and less internal damage, harder to place correctly and targets air. Maybe not worth for +1 elixir. But situational-dependent.

Inferno Tower(A): Now we're talking! Destroys tanks but can be reset by Zap, Freeze or an Electro Wizard. Only 5 elixir, but makes sure you still have enough to counter the remaining troops.

Hog Rider(A): Great pushing unit with high damage and health, and moves very quickly. Paired correctly and your opponent will be destroyed. Most decks with this are suitable for 1-crown wins only.

Freeze(D): Not a big area of effect, and costs 4 elixir. Handy for stopping some units sometimes though.

PEKKA(D): You'll find it hard for this to reach the tower, unless you are facing some noob. 3 seconds deploy time makes it useless for defending as well.

Inferno Dragon(C): Unless your opponent cannot counter this, you may find it hard for it to reach the tower as well. Defense- use IFT instead.

Lava Hound(C): Only if paired with stuff like Mega Minion and clone will this be good at all.

Spell Valley:

Zap(A): Instant killer of anything up to Goblins, no deploy time at all except for the universal 1 second delay thingy. Only 2 elixir.

Fire Spirits(B): High damage against swarms and can be used to pig push, destroying any Minion Horde or Minions along the way.

Furnace(B): Good spell bait or face chip damage! Also handy for defending Hog pushes.

Wizard(D): Okay in essentials except health. Killed by Fireball, Lightning and Rocket.

Poison(D): Does not kill Skeletons in one second and troops and just walk out of range.

Mirror(C):Situation-dependent. Punishing if the opponent can counter both original and mirrored copies.

Ice Wizard(C): Slow effect is annoying, but damage is way too low.

Graveyard(A): If used correctly, a single push an severely damage one's tower and can get clutch wins.

Builder's Workshop

Mortar(D): Can do some chip damage if opponent is inexperienced, but damage sucks anyway.

Elixir Collector(D): The elixir advantage will not be very obvious and may not do much at all, as 2 elixir is easy to lose to th opponent.

Battle Ram(B): Real pain for those who can't counter it well, but can be shut down easily by a number of cards.

Golem(C): High health and spawns distracting Golemites when destroyed. Can be devastating if used correctly or used in double elixir time.

Tornado(B): Damage is ok, especially against cloned units and also swarms. Pull effect is also useful to activate King's Tower early.

Miner(B): For some reason many people use this card, which is a good tank at 3 elixir and can destroy Elixir Collector, Princess and also chip the tower(not recommended). But king activation is oftn due to this card.

Sparky(E): Reach the tower? No, I don't think so. Easily counterable for 6 elixir.

Log(A): Not exactly zero deploytime, but can be a substitude for Zap. Does more damage and targets ground only. and knockback. Very useful actually.

Royal Arena

Royal Giant(B): You can never prevent damage to the tower. Especially useful when you have taken a tower already. You can surprise your enemy by palcing it in the middle of your new territory and do lots of damage to the second tower.

Elite Barbarians(A): Each hit does the damage of a Hog. Not vulnerable to Fireball but still will be weakened. Moves quickly and if paried correctly, can reach the tower and do lots of damage.

Three Muskets(D): Counterable by Fireball, Lightning and Rocket. Unless your opponent has nothing to counter, useless.

Guards(B): Damage and speed are definitely not as high as Goblins, but their shield protects against all spells and also has a slight ranged attack like the Prince. Used to get positive elixir trades.

Dark Prince(C): Low health and damage. Not really useful unless paired correctly.

Princess(B): Targets air and does splash damage. Very long range but attacks slowly and has low health. Can do chip damage and counters swarms well,

Electro Wizard(C): Deals high damage and stuns, also does spawn damage, but has very low health. Killed by Fireball.

Frozen Peak

Ice Spirit(B): Very useful for defending and also pushing for only 1 elixir. However, it can be killed before it jumps to freeze.

Ice Golem(B): Useful for distracting and also serves as a Lightning Rod. However, it has very low damage and its death damage can't kill anything more than skeletons. Also cheap.

Bowler(A): Knockback and straight-line attack. Has high health and damage for 5 elixir.

Clone(C):Situation-dependent. Only good when used properly, and it is hard to do so.

Lumberjack(B): High damage and moderate health. Drops rage upon death. Attacks quickly and moves quickly for only 4 elixir. Outclasses the Mini PEKKA.

Jungle Arena

Goblin Gang(B): Can be zapped, and can also substitude Skarmy. Does lots of damage and you won't see any more Gobins or Spear Goblins any more.

Dart Goblin(C): Something to get rid of buildings and counter Graveyard, but 3 elixir is too much.

Executioner(B): A Bowler that has less health, no knockback and can attack air. Counters swarms very well and can tank a bit.