So, It was hard for me to not make posts by Zelefore and Zelarith as the main posts here. Good descriptions, as well as it seems many experienced players agree more, than less. So this list among the other lists here, will be very helpful for new players, along as veterans that havent been too much in the game lately, but decided to come back. Thank you all for this! 
Zelfore’s opinion:
( @Zelfore )
Tier 0:
Bone Dragon
The reason is
obvious. In the current meta of high Attack, Health, and most
importantly to this dragon, Armor, Bone Dragon destroys the foes Armor
and quickly turns it against it’s foe. Unless we see an update that
greatly lowers the armor gain from ascension/final-stage-leveling, he
won’t be losing his pedestal. Also ‘Agile’, goshdangit…
Tier 1:
Gloom Leaf
Ranked for his spell
which offers massive gain, but even more so because of the trait
‘Stoneskin’. Making an already bulky monster with a spell that increases
bulk and now a trait that doubles all bulk, it’s safe to say he’s
beyond obnoxious defensively. Thank the devs for no ‘Agile’.
Keeper of Souls
Okay defensively, great offensively, still inferior to the Bone Dragon.
Sheggra
Like Bone Dragon but with easier color denial.
Tier 2:
Carnex
Armored + Skull go boom, gotta love explosives!
Celestasia
Any creature can be fed, but only this feeder troop can feed them all!
Venoxia
Attack Boost, Self Feeds, Actually good Traits.
Tier 3:
Abhorath
Clears the board, self-recovery, sometimes backfires.
Behemoth
Good damage, poor follow-up.
Crimson Bat
Direct allout attack, only decent damage.
Goblin King
Pairs with other goblins + mana denial, what more do you need?
Gorgotha
Boom-Shaka-Laka!
Hydra
Great spell damage when injured, but masochism can go a little too far sometimes. X_X
Imp of Love
Charming, yet imp-udent. One mana type kind of hurts it, luckily it’s easily fed.
Jarl Firemantle
Self-feeder. He’s very consistent in usefulness throughout the patches.
Orion
Sucks much less than previous patches, but still sucks a little.
Shadow Dragon
Can perform an astounding trick of self-feeding, but he can only do it once.
The Silent One
Silences (almost) all of the opponents. Silences self.
Webspinner
Webspinner ain’t what it used to be. Still a self-feeder though.
Tier 4:
Autumnal Imp
Great if it kills something. Very difficult to kill something.
Kerberos
1/4 Chance of being useful.
Moloch
Drains the enemy of mana, but the cost is too darn high!
Psion
Drains mana and deals damage directly. But it’s all rather on the low end.
Summer Imp
Refer to Autumnal Imp.
Winter Imp
Refer to Summer Imp.
Tier 5:
Gar’Nok
A bad Leader, a bad Summoner, overall just bad. He does get ‘Armored’ if you really think it’s worth it.
Zelarith’s opinion:
( @Zelarith )
God Tiers :
Bone Dragon
Mana cost isn’t as high as Keeper
of Souls and Sheggra, besides, even AI can use it properly. As deadly
as they come, he almost never backfires with armor being so high when
everything is maxed out, while not requiring any board setup. Just don’t
lower your ennemies’ armor beforehand…
Sheggra
As he creates red gems before turning
them to skulls, once again, AI is quite capable of using it properly.
The downside is that he can be countered by getting rid of red gems, and
that he will cripple his own mana generation… Hopefully, there are
quite good decks to build around him that work wonders… Oh, did I
mention his number of gem spawned scales with magic?
Gloom Leaf
Stoneskin, Huge help him being so
tanky, and his spell just adds up to the tankyness while reducing
ennemies’ strength and increasing his own. This adds up o make hm the
most powerfull off tank in the game !
Keeper of Souls
Not so potent in the hands of
AI, but players will not only deal a lot of damage using his spell,
they will also deny a mana color ! Too bad his traits aren’t so great
battle wise…
A Tiers :
Abhorath
Great self-recovery capacities,
Huge trait allows him to increase his life and since he heals to full
life, he will also regenerate all the life Huge brought him ! Not to
mention he will get very powerfull from arcane combined with his spells !
Nothing spells one shot better that Abhorath that casted his spell 2 or
3 times. The downside is his random board cleaning that will sometimes
backfire… But sometimes bring great things too ! Might turn out to be
great with the incoming frozen effect… Maybe.
Celestasia
One of the best color spawner of
the game : we are usually limited to 9 gems spawned, Celestasia is the
only one spawning 10 gems (except when gem-spawn scales with magic).
Especially good at the moment since some common cards with only one
color are meta : having control of what gem colors you are gonna spawn
is quite efficient with Celestasia. Might be good combined with some
imps though, especially with Imp Of Love since it’s one strong Imp !
Carnex
Armored adds up the tankyness it gets
from its spell. Explosion helps charge others while dealing little
damage. Too bad it is limited to skulls : once it uses its spell, itwill
get rid of a lot of skulls and, although it might self charge from its
spell, it won’t be able to use its spell at full efficiency because,
sometimes, there won’t be much skulls left on the board.
Gorgotha
A lot like Carnex, without the
tankyness, but without the limitations to its spell. Also its magic link
helps him charge faster (though it doesn’t help him when he uses
explosion).
Imp of Love
Good damage, its spell sure
works… Like a charm ! Too bad it only uses one mana color, but it has
its perks too as I mentionned earlier, speaking of Celestasia. Might add
in a hero using Prismatic Orb. If you wonder, it also gets its +8 magic
if you kill an ennemy thanks to the charm effect, not only from its raw
damage. So, the more ennemy there are, the more damage you are likely
to do through charm, and the less there are, the more you do to a single
target with the split damage!
Webspinner
Self feeder, quite big damages
plus poison everyone, can keep a green combo (goblins…) running. Not
realy fun to play, but defense decks can work around it and end up being
relatively good.
Jarl Firemantle
A good leader, his spell feed
himself but also feed his huge trait ! There are some interesting
combos to build around him including Valkyrie, and that is quite good
when we know how our soul farming depends on Valkyrie…
Shadow Dragon
I see some good combos with
Shadow Dragon, sure, his self feeding won’t help him a lot since he’ll
loop only once, but get him behind another purple user, and he will feed
that guy instead of himself. Same thing could be said of other self
feeders, but since Alchemist will set up your Shadow Dragon so easily, I
think he is worth A tier. Even if his traits aren’t worth unlocking
(except the first one).
B Tiers :
Silent One
Silence all is quite potent, but
the guy needs so much mana and will silence himself. Besides, there are a
couple cards with immunities to silence… Traiting him won’t change
much…
Hydra
Great split damage with a big ratio on
its missing health. And it get more health with the Huge trait. That’s
quite the high risk / high reward kind of creature. Can be really strong
in the hands of a player, but I don’t think an AI will use it right.
Venoxia
Self feeder that also feed other.
Gets a lot of attack. Can combo well with Webspinner or Green Slime, but
in my opinion, it falls behind Jarl Firemantle : self feeders are used
for well rounded spell loops. Gaining attack in a spell round isn’t very
helpfull, and the poor true damage it does was really great a long time
ago, but not so much anymore. So I kinda don’t like it… Now it’s one
of the few Dragons in the game, and the only one using green mana so it
still is interesting.
Kerberos
It’s not really that strong, but
you’d just hate to have him in front of you with it’s spell prepared to
be fired. If you are unlucky enough, you’ll end up devoured. That can
break combos all while renforcing the ennemy. Even without devour,
Arcane can make its spell frightening, but other cheaper (mana AND
traitston wide) units scales better with Arcane.
Moloch
Deny ennemy spell and turn their mana
against them ! Could be good in a true damage deck, though his mana
colours doesn’t open him up to many combos… Well, it’s the only card I
miss so I might over rate him…
Psion
Prevent ennemy from casting spells.
Draining 4 mana also prevent them from getting their spells back with a
3-gem match unless they have a mana surge. But you’ll need to come up
with a well rounded combo to prevent your ennemies from spell casting !
Orion
Damage is too random, Hunter Mark is a
good addition… Well, the only reason he isn’t C tiers is its trueshot
trait… But there are better solutions at lower rarity
Behemoth
Good spell damage, a bit of random
mana generation, and combos well with Terraxis. A bit sad he is
classified as one of the few monsters while there are so few (good)
monsters in the game, and it doesn’t combo well with them, nor with any
prideland units.
C Tiers :
Crimson Bat
Good true damage on all,
replenish its own life but… Needs a lot of mana, no good combos, no
utility in breaking your ennemy combo or preparing your own…
Autumn Imp
Only one mana color user, a bit of
mana generation, scales as it kills with its spell… But as trouble
killing with its spell in the first place ! I guess he could have its
place as a finisher in a Terraxis / Behemoth combo
Summer Imp
Same as his autumnal brother, but
can combo by creating red gems… Low chances to combo though, and might
backfire. Good with an Alchemist I guess, but I can’t see a deck build
around this card
Winter Imp
He only uses blue mana… And
remove all blue mana to scale? Well, it could be really good with
Valkyrie if at least he scaled well with the blue gems he removes, but
he don’t ! So he doesn’t synergize well with anything… Too bad!
Goblin King
Leader + Summoner = error. Besides, he summon such
an underwhelmingly low level unit, you’ll let that goblin live and
focus your spell on the other units behind it. The only good thing about
goblin king is his health and his mana denial… Oh, and it’s a goblin,
so he is usefull in a goblin deck I guess…
Gar’Nok
Again, Leader + Summoner = error. At least, he summons
a real unit, boost everyone’s attack drastically, but there aren’t any
real good combos with the fact he hurts his allies : Hydra? No, they
both need red mana so the combo is kinda hard to setup. Frenzy trait?
No, he already brings a lot of attack himself, no need for a poor +1
from Frenzy.
