So....there are 20 Common cards in the game, here I'm going to rank them from worst to best.
PLEASE NOTE, THIS IS JUST MY OPINION!!!
20. Spear Goblins
I don't know what is really good about these guys. They deal almost no damage, now Archers are better in many situations for 1 extra Elixir. These guys almost can't do anything, they die too easily, I barely see any of them in Hog Mountain.
Honestly, Tesla is pretty weak for a 4 Elixir building, Cannon is better in much more situations since it is cheaper, and most of the time enemy's pushes are ground based. Sure it can target air, but you can just have better anti air units in your deck. Just like Spear Goblins and Goblins, I barely see Tesla in Hog Mountain.
Ever since the arrival of Goblin Gang, this card is really underused. They still provide some value for 2 Elixir with their high damage, but Skeletons are a much cheaper distraction. And Goblin Gang is absolutely better for 1 extra Elixir.
Supercell had gone too brutal to the Cannon. Now Cannon is pretty weak, although it cost only 3 Elixir which is so cheap, Inferno Tower is better in killing high hitpoint troops (All buildings except Inferno and Furnace are underpowered actually).
Mortar is pretty decent. It is like less risky but less rewarding X-Bow. But I don't really see any of its defensive value. Still a good card on offense if you can protect it with many things.
Bomber is really fragile, but for 3 Elixir, it can provide good value on defense and support since it deals massive area damage in big radius. But there are just better splash options to play.
The loss of Ledoot yet again reduce Skeletons value greatly. Still great as a cheap distracting unit and cycle card. But not great as before. Gosh, if Supercell just leave the Skeletons alone.
Flying Skeletons :P. Bats are actually decent for 2 Elixir, they can deal good DPS, and also a great Zap bait. You can use them indirectly via Night Witch.
12. Royal Giant
This card is very MEEEEEH at tourney standards, people just use this card because of overlevelling. It barely damages Arena Tower for 6 Elixir card, and any type of high damaging cards can counter him with minimal damage suffered to your tower.
Barbarians are great tanky swarm. They can deal with almost any ground pushes, while very resistant to spells like Zap and Arrows. Really good, but they are useless on offense and counter push.
Arrows are decent spell, they are really great against swarms since it can kill them instantly. Although outclassed by popular Zap, I still think this is a great spell.
Archers are great for just 3 Elixir, they can provide good value on support and also defense. Can deal serious chip damage when ignored. They also far more resistant than Spear Goblins to some spells.
8. Elite Barbarians
This card is pretty mediocre in tourney standards, in ladder they are good when overleveled. Almost any combination of cheap cards + high damage cards/splash cards can counter them efficiently. They best used in counter push, but still can be countered fairly easy if you have the right cards.
7. Fire Spirits
This card is really good. They are like smaller Fireball split into 3. They can deal serious damage to glass cannons and completely wipe swarms. But use them carefully since they can die too easily.
Minions are great defense and support troops especially for beat-down pushes. They can counter almost anything ground and air based with just 3 Elixir. They can distract Inferno Tower for your push, etc. A great card overall.
5. Minion Horde
Minion Horde is really great defense card. They can defend against many things and even survive for counter push. This card is a monster in support, but they are fragile to Arrows. High-risk and high-reward card.
4. Goblin Gang
This card is extremely powerful for 3 Elixir, even after they lose 1 Spear Goblin. They can be used to stop many single-target troops. And great support for your push. Also makes a good Log bait.
This card is super versatile. Lots of health for just 3 Elixir, many defensive uses, good meat- shield in offense. He may be outclassed by other mini-tanks, but he is the cheapest one with good amount of health. I love this card!
2. Ice Spirit
I have no doubt putting this on 2nd place, this card for 1 Elixir has many good uses. Cycling, defense, and support. Saving your towers, freezing defensive building/troops. There are many uses.
2 Elixir, so cheap, so weak, so versatile. Zap is almost a must-have-in-every-deck card. For the cheap cost, it can stun troops, reset charges, make troops retarget, cripple if not kill swarms. Many uses for just 2 Elixir.
That is all. Let me know about your opinions :)