There are a lot of Common cards in the game. Here I rank all the Common cards from best to worst, according to my opinion, research and experience.
The Goblins are the worst Common card. Why? They are essentially stronger Skeletons which cost 2 Elixir, but they have no real use. Skeletons are a better distraction, Spear Goblins have range which makes them just as effective and Bats have the same damage as Goblins but are flying, for the same cost. They can be swapped for another better card, no matter what you use them for.
20: Spear Goblins
The Spear Goblins are the second-worst Common card. The Bats just do their job better, and they are spawned from the Goblin Hut and Gangs with better ability. They also have low health, but have a better chance of killing things than Goblins, which ranks them in position #18.
The Tesla is, in theory, OP. For one more Elixir than the Cannon, you get a building which does more damage, attacks everything and is invincible when not attacking. The harsh truth is, it's too easily distracted by air units and dies immediately to strong units and swarms.
The Cannon is a ground-targeting Tesla which can't hide and does less damage. It still is better because it's cheaper and gets less distracted by air units, making it better able to kill important units (because the strongest ones are often grounded). It still has low HP and dies practically immediately to spells. Some, like me, may prefer the Tesla because they use the Tesla for the things completely unique to it, but overall the Cannon is better.
A splash-damaging unit which can hit from miles away. It sounds OP for 4 Elixir, but its blindzone makes it too vulnerable. It has the hit speed of a Sparky, and requires very specific decks to work.
The Bomber does immense ground splash damage and has really low health. A simple Zap or The Log (or even Skeletons) will shut it down by allowing other units to shoot it down. Its one redeeming quality is its ability to defend because of the amount of damage it does.
Back when Ledoot was around, the Skeletons were ranked #1 on this list because of their ability to cycle, distract and shut down power units like the Ice Golem, Miner or Mini P.E.K.K.A.. Now that Ledoot left, though, there are now three Skeletons. They are less able to kill and distract and do their job. The Skeletons still do their job, they just do it way worse now.
14: Skeleton Barrel
Right now, this card isn't out, but it has low HP and drops low HP troops, not doing any damage of its own. Get it to drop in the right place, and it's OP. Get it to drop anywhere else (which happens way more), and it does absolutely nothing.
13: Royal Giant
Giants with less HP and a cannon get you hate from the community. All the Royal Giant is is a cancerous way of getting extra tower damage off, with his cancerous range. But an Inferno Tower, Bomb Tower or even Tesla will shut it down. It dies so quickly to the majority of cards, as no-one ignores one of these, and cards will pile up on it until it becomes unable to do anything. It's good when it's overleveled, but in Tournaments, it's terrible.
They have 2000 HP in total and do tons of damage. They aren't good on offense at all, which makes them a 5 Elixir purely defensive card. They savage other cards on defense, but once they cross that bridge, they're dead. This is one card that their dropper does not want crossing the bridge.
The three Minions are effective on defense, being able to destroy swarms effectively. They aren't good against other troops, because of their low count and health. The Mega Minion is better on offense, and the Minion Horde is much more costly but much more rewarding than this low-risk, low-reward card.
Archers have the same problem as Barbarians. They're purely defensive troops, only able to do chip damage to towers. They cost less and have range, which makes them better than Barbarians, but they can't really do anything else. They still do love to defend the Golems sometimes, so they're not dead yet.
Simple yet effective swarm clearer. Such a godly card in general, and can spell-cycle towers to death quickly. Don't do much damage and have no additional effects for 3 Elixir, however.
8: Minion Horde
These six Minions are an absolutely savage card if you get them to the right target. Mess them up and they do nothing, and they can be easily killed by Arrows and even a Zap/Tower combination, which makes them a rather risk/reward horde.
7: Fire Spirits
They dive-bomb headfirst into enemies, doing explosive splash damage. Basically cheaper, deadlier Arrows which can die and must be directed to the right target to do damage. They do cost 2 Elixir, which negates a lot of potential risk. Also, they do insane damage when all three hit.
6: Elite Barbarians
A cancer card which is very fast and has way too much HP and damage. On your side, they can take out any single ground troop that isn't a PEKKA or Sparky or Barbarians or other Elite Barbarians before it gets a hit off on your tower, and believe me, this list of targets goes on and on. They are also extremely useful rush cards, and a hand of spells coupled with these guys gives you a basically unstoppable push. They still have counters in spells and air units, so they aren't 100% infalliable. Also, they get you a lot of hate from the community.
He has so much health for 3 Elixir, and moderate damage coupled with a rather fast hit speed. A cheap all-rounder who doesn't master any role but can serve any. Again, air troops will demolish this card, but he's good for overall making your opponent weaker so that the rest of your troops can finish your opponent.
They are the old Skeletons but airborne and cost only one more Elixir! The Bats are a fantastic card, able to take out nearly anything with the right placement. They are also a high-damage card and are very powerful on defense if ignored. They die to one hit from most things, however, making them an easy-to-kill pest card.
3: Goblin Gang
This card is purely fantastic. It can surround and shut down most troops like Royal Giant, Elite Barbarians and similar troops. Its Spear Goblins also help against air troops and the Goblins help do insane damage to other threats. They are also a great rush card for early-game. Basically a more offensive Skeleton Army.
2: Ice Spirit
One little troop that does explosive splash damage which is really low, and freezes for quite a while for 1 Elixir. It also cycles the deck well and can attack air. One Ice Spirit can make the difference between winning and losing, and it is the better of the two 1 Elixir cards right now.
The Zap spell is still the most useful and best Common card around. It's a more expensive Ice Spirit which has instant effect, stuns instead of freezes and does more damage. It obliterates Skeleton Armies and similar cards, resets the deadly charge cards (especially the Infernos) and can make the difference between winning and losing through the small stun it does. Congratulations, Zap spell, for being the current best Common card!