Any minion not listed here did not receive enough votes to achieve F+, and should be considered F tier.
Commentary will be provided for the units B+ tier or above.
Frost Mephit

To get an idea of how strong Frost Mephit is considered, it's the only piece in our dataset that got an S+ rating
for reference, if everyone voted for something as 2nd, that would be the border between S and S+
So an S+ rating reflects near unanimous agreement that a unit is strong, and many first place votes.
First of all, its moveset is very good, much better than that of skeleton.
It has two forward attack squares, which is the most important type of attack squares. Placing two of them next to each other, side by side, forms a wall in front of it. Compare that to Skeleton, which is easily annoyed by Lilith, Pyromancer, and Soulkeeper.
The diagonal attack is also superior than orthogonal attack when dealing with Samurai, Hoplite, Fencer, and Crusader.
Being able to retreat allows it to avoid freekill, as well as avoid trades against the few things that uptrade with it.

And that's not getting into its special freeze ability, which is also very relevant.
You can "hang" it in your second rank and not worry about spending turns to defend it, because anything that's fast enough to reach it is also valuable enough to be punished by the freeze turn.
It also makes for excellent king defense, particularly against armored units.
A lot of great qualities at a low cost of 2. It's hard to go wrong with Mephit.
Mercenary (Guest Commentary by Runner16)
Mercenary is a good piece overall during all the phases of the game, although it excels in the early game. Its main objective is to exchange against higher value minions (in which both its moveset and cost make it very good considering some of the best minions are very weak to it) but it can also be used in 3 for 1 trades against champions. This second option should be reviewed carefully as giving mercenary to the opponent can lead to enough compensation.
Its non entropic nature makes it specially good in armies like stall and midrange with a high focus on trades, but it is slow and doesn't have a good spot in any type of rush.

The first thing that you notice with the merc is the drawback. The thing is, most of the time, the drawback doesn't actually matter.
If you take a minion with it, it literally doesn't matter.
Meanwhile, when going after most champions, taking the champion and giving the merc is still a good trade for you. Because of this, your opponent will still usually be forced to move the champion out of the way, which would be the exact same result as if you used a skeleton+.
That said, it's not advised to use too many of these, because there are cases where the drawback does matter:
Firstly, it can't provide good anchors for your champions. If you were to move a medusa into the center of the board with only a merc defending it, your opponent can take it with a rook.
Because of this, if you have too many mercs in your minion line, you will have trouble developing.
The second thing is it has some issues on defense: If your opponent has two rooks attacking something, and its defended by a mercenary and king, then even though you have two defenders to two attackers, you may have an issue.
The typical solution to this is to put mercenaries in the center and high value units in the corners. This puts an upper limit on how many mercs you can use in an army.

Samurai
It's by far the most popular piece on ladder, so every army should be prepared to deal with it.
So sam controls all 3 squares in front of it, and controls one of them strongly
its the cheapest playable unit that controls those 3 squares
makes sam walls hard to attack from the front
its non-trivial to approach sam walls with merc and mephit, and drake downtrades into it
i think this is part of the reason it needs specialized play against
vs royalguard++ (a more expensive unit that controls those squares) wall you can just break it by throwing drakes, for example
But it's actually in some ways scarier than even that.
Consider two samurai side-by-side
At any moment, one samurai can move in front of the other, making a fork. Furthermore, unlike a normal fork, it can't be dealt with by just attacking the forking piece, since the back samurai can auto-attack, then attack again on its turn.
So the samurai "brick" basically makes it extremely dangerous to have high value pieces near it, which is what makes it so scary
That said, samurai does have a few weaknesses.
Mephits can generally force trades by hitting from the edges.
Rangers can zone and occasionally freekill it because its entropic.
Infiltrations such as from pyromancer and the like are extremely dangerous so samurai players need to be very careful to avoid them.

Drake (guest commentary by f0x):
Drake is more of a miniature champion than a minion - it costs 4 so it is more expensive than most other minions, making it unprofitable to trade with them. Due to this, running a large amount of Drakes is inadvisable. Drake's primary purpose is to threaten champions while being defended by other pieces.

Drake typically has limited influence in the early-game because players typically start the game by developing free-kill pieces that Drake cannot offensively threaten, or minions that cannot be efficiently traded with because they cost less than Drake. Drake can be used to "perpetual" (initiate a repetitive sequence of moves) free-kill pieces in the early game, but going into a "perpetual" as the defending player will usually result in you losing the game.

Once most enemy minions have been cleared off the board, Drake excels because of its cheap cost. There are no champions that can trade profitably with Drake - the cheapest champions cost 4 or have an associated morale penalty. You can constantly threaten to make profitable trades against enemy champions by threatening them with Drake and defending the Drake with your other champions.

Butterfly+ (guest commentary by Firemorfox)
at first glance, it looks terrible:
-it's a skeleton0 without the status effect,
-its ability means wasting your tempo, sending the butterfly+ to a square where your opponent has 20 turns to kill it for free using: samurais, antimages/lifestones, stone pillars, or cheap 0-1 cost minions.

However, it's an incredible tempo unit because:
-it has range4 teleport/kill
-it can threaten backrow enemy champions or king and ignore almost all defenses they have, rare exceptions being vampire, soulkeeper, and lilith.
-it can threaten up to 5 squares using only one move, as well as open up space for allies such as bishop0 or phalanx++
-turns in the beginning are typically low value and low-pressure, while turns in midgame/endgame tend to be extremely tight on tempo.
-armored and ranged units, or trigger lust/samurai units, can really abuse the tempo provided by butterfly+
Typical use-cases:
-use an army with extremely mobile champions. Common synergies include:
fencer+, medusa0, lust+++,
-use an army with armor or high amount of tempo threats:
firemage+++, wrath+, duelist0/+, hauntedArmor0, samurai0, pawn+
-use units with unilateral threats that retain tempo after threatening the opponent:
ranger++, ranger+, envy+, envy0, temperance0
-use an army that easily threatens undercosting your opponent
queen0
-use an army that has unblockables to threaten king, while your butterfly is about to spawn in and threaten an expensive enemy champion.
apprentice++, ninja0, angel0, angel+++
-windmage+ can be surprisingly effective by surprising your opponent with wind-pushed ranger, samurai, envy, or other highly square-sensitive but slow-moving units.

General use example:
-against typical armies, move butterfly+ up for the first 3 moves. Cast it to threaten 3 backrow squares, typically favoring the side closer to your opponent's king.
-use a wrath+ tempo spam checkmate army.
-continually apply tempo pressure with fencer+, ranger+, envy0/envy+, and firemage+++
-assuming you manage to trade 14-15 of your turns 1-for-1 to your opponent, butterfly+ will threaten to gain material, or your opponent's defenses are extremely scattered.
-force your opponent to retreat with undercost threats from armored units (fencer+, hauntedArmor0, phalanx++, phalanx++)
-aim to checkmate your opponent's king using armored units.
RG++ and RG+++
Being the most expensive units in this list, they've fallen a bit out of favor as of late due to downtrading with other minions.
That said, kingmelee is quite a good moveset, well worth 5 points. In addition, it makes attacks with queen that much more scary, and can serve as an additional defender on the king.
Duelist and Duelist+
Although they have only one threat square, it has armor, which means it can poke units from "undefended" spaces, and good mobility means it can easily get there. It can force trades with minions, even samurai (if you're careful). It also has a promotion threat, and even though the fencer isn't very high value, the fact that it starts with armor makes the promotion threat difficult to defend against.

Reaver++
Like said above with the mercenary merely controlling the space does a lot to deter opponents from going there, and reaver controls a ton of space for 5. If you advance it, your opponent either has to let you control a ton of space, or trade into it, making the penalty meaningless.
Pawn and pawn+
While vulnerable to a ton of things (ranger, samurai..), it controls two forward spaces, and does so very cheaply and very quickly. Advancing two pawns next to each other is a very quick way to take control of the center.
Hoplite+
Is its own army, but a pretty scary rush at that. See e3's guide.

this needs cleaning up, the butterfly section seems too verbose
@awoodokehu thoughts

HA

• + generically best 8-drop in the game
  
• + pretty much always uptrades
  
• + excellent at bullying enemy rangers and FEs (though this army already has tools to do this)
  
• - slow to develop, and limited ability in providing anchor points for ranger
  
• - best in endgames, and the goal of stall is to never play an endgame
  medusa
  
• + generically best 9-drop in the game
  
• + good board control for its cost, and excellent at poking away the likes of SK
  
• + nullmage can fix colorboundedness
  
• - petrify is sometimes pretend
  Guardian+++
  
• + Sick queen++ synergy
  
• + Can stop opponent from trading ranger (and protect it vs summoner/nexus, which it is normally pretty vulnerable to)
  
• - Mediocre board control on its own, especially compared to the others on this list (which tend to be premiums at their price point)
  lust+++
  
• + generically best 10-drop in the game. at least 1 undercosted, its nuts
  
• + if ranger gets traded, it can sometimes function as the second copy
  
• + you can ruin hoplite formations
  
• - vulnerability to mephits and double attack threats mean i'd never run more than 1 in stall
  temperance
  
• + Good at zoning ranger, sk, and especially phal which stall sometimes struggles with
  
• + Provides anchors and an extra king defender (with side attack) without need for development
  
• - value loss may cause issues in decay endgames