There are a lot of Rare cards in the game. Here I rank all the Rare cards in my opinion, from worst to best.
22: Flying Machine
The Flying Machine is the worst Rare card. It's just a Rare Baby Dragon with more range, the same damage, no splash and that can die to Fireball. Use Baby Dragon instead. The extra range doesn't make this card better, as it can't survive nearly long enough to do good damage.
21: Barbarian Hut
The Barbarian Hut is really useless. Sure, it has a lot of health, but it spawns Barbarians way too slowly for defense. And they were built for defense, not for being pumped out from a hut on offense. A spell can easily shut this down hard. To work this card, you need a very specific deck and a game that goes your way. Which is really rare.
20: Goblin Hut
The Goblin Hut is a chip damage card. It sends extremely fast Spear Goblins, but the element of surprise is its only advantage. Anything that is placed down can stop the Goblins from doing tower damage, and anything that attacks them will kill them. Also, a spell downs it so easily.
19: Bomb Tower
The Bomb Tower is a stationary Executioner which throws faster but can't hit air and loses HP over time. Use the Executioner instead.
Heal is a great idea which often doesn't work because the units die so quickly there is nothing to Heal. Time it right, though, and the Heal is an absolute beast of a card.
17: Ice Golem
The Ice Golem is the Rare equivalent of Skeletons. Before, it was a beast of a card found in every deck because of its crowd control ability. Now, it got nerfed so hard that it's underused. Skilled players can still make it do its job, it just can't do it as it used to.
She's seen quite a bit, but she's bad. She dies to an equal-level Minions or Mega Minion. She doesn't do splash, and she has so little health that she dies to a Fireball + anything combo. There are far better options than her.
She costs little Elixir, does 360 splash and has a lot of health, but really she's a splash tank that can be replaced by other cards, such as the Baby Dragon, an air splash tank. She's also easy to kill. One Fireball will take her health down a bit, and from there she's so slow that she's rather easy to kill.
14: Mini P.E.K.K.A.
He's actually a really good card for his cost. It's just that there are better options than him, like the Night Witch, a Mini P.E.K.K.A. that spawns Bats, or the Lumberjack, a faster Mini P.E.K.K.A. that drops Rage.
The Wizard is actually rather powerful to some people because of the damage he does. He's better for damage than Executioner but worse at surviving Lightning, which is a key factor for splash units.
SUCH AN ANNOYING CARD AGAINST LOW-HP TOWERS!!! Really hard to aim otherwise. Unloads a ton of damage but is just really bad against spread targets.
He's got so much HP for his cost, and a lot of damage too. But recently, Beatdown has been rising, and Golem is much better than Giant for the job. It does death damage and has Golemites, and is also much harder to kill than this card. The Giant is an easy kill once it's on its own, but is really good with other cards.
10: Three Musketeers
The most expensive card. Played right, they can really catch an opponent off-guard and demolish a whole push by themselves (even ones with Golem in them). One Fireball + anything, and they all die for a major Elixir profit.
9: Battle Ram
Does tons of damage when it connects to the tower, and only attacks buildings. Does no damage when it doesn't, and has less HP than a Hog Rider. The extra Barbarians won't help, as two Barbarians die very quickly. A more threatening but easier to kill Hog Rider.
8: Dart Goblin
On defense, this card is a beast. Its range and damage help it weaken any push to the point where it can be cleared. Spells, however, kill the Dart Goblin before it can do anything, and also harm anything around it. It is also insanely good at chip damage, though, as it can fire three darts before it dies to the tower. For 3 Elixir, that's not bad at all.
The classic spell. Does a lot of damage and knocks back, but can't completely finish off most of the current meta. An Ice Golem or Heal or anything similar can save any card from the otherwise deadly Fireball, which its only bad point, and why it ranks #7.
A building which quickly spawns Skeletons, which accumulate really quickly. Is useless on offense. Ends up getting killed a lot on defense, which works to its advantage, since the on-death Skeletons will add to the pile on whatever the Tombstone's target is. When Skeletons stack up like that, they become deadly. It's also an amazing single-troop counter (especially those like Hog Rider).
A building which slowly spawns Fire Spirits. Has a reputation as a cancer card, because at appropriate level it will still be able to get one Fire Spirit to the tower with each wave. A spell like Lightning takes it out, and Fireball cripples it, but its cost of 4 Elixir makes it an invaluable building to have in the deck.
4: Elixir Collector
It's all about dropping it at the right time. If the Elixir Collector can survive a while, your Elixir advantage over your opponent will build to such a level that they will not be able to do anything against your onslaught, fueled by a constant stream of Elixir. It also lasts a minute and ten seconds by itself, so ignoring it is not advised.
3: Mega Minion
The Mega Minion has enough health to survive a Fireball, and also dishes out major damage on either side. Great at clearing away Executioners and other potentially deadly troops, either to back your Golem or to not let your tower die.
2: Inferno Tower
With the rise of the building-targeting units with high HP, the Inferno Tower is a must. It starts off small, but does insane damage over time. Just pack an Ice Spirit in case it gets Zapped, to give it time to defend itself. Also lures units that target buildings, and has so much HP it can survive Sparky and stall games so that you can draw instead of lose. Very weak to swarms, but one simple spell will easily destroy them. Great card overall.
1: Hog Rider
Who wouldn't love a card that's as fast as a Goblin, targets only buildings and does the damage of a Giant? This card is so fast and has tons of health. So much that a Zap or Arrows has enough time to get to the swarm that's attacking it before it dies. It only attacks buildings and does more than 210 damage per swing. It also jumps the bridge, and only costs 4 Elixir. It's very easy to lure it with buildings which can be taken out by a spell, leaving the opponent at the mercy of your Hog. Certain units like the Wizard or Guards will do enough damage to not let the Hog Rider get too many hits off, but this card is the best of the Rares for how quick and easy it is to use. The Hog Rider is currently the best Rare, and it intends to keep it that way.