Tier lists are admittedly tough. Especially with balancing passes fairly frequently. On the other hand, there is value in understanding the tiers and (more importantly) why they are what they are. This is of course my opinion, but I don't think much here is going to be controversial. This is for Mythic play, which is a lot different from lower- and mid-ranks.
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All creatures listed left-to-right and top-to-bottom in rank order (top left = best). There are more creatures at each mana slot than I've listed, but if they are missing, it means bottom tier. A special note about amazons: if building an amazon deck, you want amazon cards, but these decks are not high-performing right now (wait until core rebalance).
A staple in any nature deck. Auto-include status. There are very few things on turns 1-2 that can kill 4 HP. The Walker on turn 1 is always a solid play and can help generate favor by attack the enemy god or helping deal that 1 extra damage to clear creatures. Regen gives it extra sustain. An excellent target for nature's many buffs.
Why does it have regen 1? No idea. It's a super-powered shieldbearer, though, and that's good enough. Turn 1 Walker + Chiron is great. Don't underestimate its relic removal, either.
Best in mana slot. Maybe best bang-for-buck of any nature card. Its first turn attack is truly random. It can hit hidden creatures, ordered creatures, backline, and ignores frontline. Has enough HP that it often survives the initial attack. This card almost always costs your opponent more than 2 mana.
Fantastic card. Can generate a lot of value in the right situation, but even a 2 mana 2/3 isn't terrible. If you are frenzied, Finnian gives 7 stats for 2 mana (2/3 Finnian + 1/1 Vibrant Fruit). Assuming the apple boy buffs another creature, that's another 1/1 stats. All that for 2 mana.
Jag is solid, but not very sexy. When looking to cut from the 2 mana slot, I usually start here. Gloam is OK, but really only helps if you have board control.
Top tier cards at 3 mana. For an aggro deck, Stag's card generation makes it the better choice. For a mid-range / Big Beasts deck, the Bear is the better play for raw stats. Yes, the extra 1 strength is almost always worth the confusion. Short story: confusion is widely regarded as -0.5 stat points, which makes the trade +0.5. Longer story: there could be times where you really need him cool and collected - namely if both boards are wide and your opponent has no frontline creatures.
I love the goat. At least when it works. Unfortunately, Mythic opponents make short work of a poor, confused creature with 3 HP. Deception and Magic are particularly unforgiving to this little guy. The Hound is also decent, but both of these creatures have to stick around a turn for their special abilities to pay off. 3 HP makes that hard. (Hound is currently slated to become 3/4, though, which will help a lot.)
Lynx in a hyper-aggro deck (supported with stag for more card draw) is fun, but not too competitive. The mythic decks that I see running it only ever have 1x. Gersemi is worth a mention, but never sees play in Mythic. 2 HP is a joke, but she is changing to 3/3. Her special only helps if you already have control of the board (aka winning).
Where's the beef? Right here muchacho. With the Moon-Crazed Cyclops nerf, this guy is back on top (it's close though). Using a bag to play this on turn 3 puts your opponent in a very difficult situation.
Arguably better in wide decks, but protected by 1-2 big bodies is good, too. Removing confused from nature creatures gives back that half stat and makes them truly over-powered. Then 2 directed damage each turn on top of that to either keep board control or deal face damage is amazing. Backline and ward make her hard to remove.
Solid card. Base stats worse than Rhino with the potential to grow into a bigger threat. Unlike Death and War gods, Nature doesn't have an auto-frenzy button. When you are winning, you want Cyclops for the snowball effect. When you are behind, you want the Rhino to help you turn things around. Tough call.
Nefarious Briar punishes relics, but is rarely a better pick over the others. Maybe Rhino + Briar in a relic-heavy meta. Bladefly is good when paired with wildfire/firewine. Starving Sabertooth can appeal to people who are sick of confused creatures, but the regen ability isn't worth much and, with armor around, 5 damage is much (much) better than 4.
Best in slot. Hope you can afford her. Spits out a 1/1 walk plant each turn that heals your god for 4 HP when it dies. Aggro War's worst nightmare. Makes it so much easier for Nature to reach mid and late game if desired.
Big and blessed. Guild Enforcers tremble at the mention of the Moose. If it's weekend ranked time, these guys are definitely included.
The poor man's Avatar of Nature. Currently scheduled for a buff in the core set refresh (puts a 7-mana wild creature in your hand on death). A decent card as it is now, but overshadowed by Moose (hence the upcoming buff).
Manticore as the top end of a hyper aggro deck might work, but probably not. Tainted Treant is OK for card generation, but random cards can be crap and the stats are incredibly weak.
Where the most arguments might arise. Nature has a lot of good options at 6-mana. Please don't try to run 6, 6-mana cards in your deck. You've got to pick 2 of these guys (max) and run 2x of each.
Undisputed current king. Amazing card. Blitz guarantees it makes an impact on the board on the turn it's played. Often swinging the balance. Armor is an OP mechanic, and even 2 can frustrate so many cards.
Former best friend at 6 mana. The extra 8 HP helps it stick around, but lots of hard removals make it risky. If it does survive, you're hoping to hit the weakest creature on the board usually. This conserves your HP while overkill spreads the love around.
Not a bad card, but a little slow and awkward. You play it, wait a turn, summon a creature. At 6-7 mana, Agrodor Protectors don't live long. Tainted Treants aren't stellar. Vibrant fruit is even slower. You need another turn to sacrifice them so they buff your big creature. Once it gets rolling, this card is great. It's just that getting rolling phase that hamstrings it.
None of these are bad cards. Nature just has a lot of good options at 6 mana. Black Rhino might have some value in a slow, control deck. Frontlines can help a lot.
Is there another creature that can go face first turn? I don't think so. Like the Pangolin, it makes an impact on the first turn it's played. It's attack is similar in concept to Underbrush Boar, but can go face and cannot attack hidden/order creatures or bypass frontline. Also, not
being confused is a big deal at 7 mana (and with 7 strength). At this stage of the game, your opponent is (hopefully) at low health. Charging Oryx can be relied on to finish them off if not dealt with.
The hydra is a fun card but doesn't see a ton of play. Mythic players know to ignore Hydra Heads until the main beast is dead. Not terrible, but you can't really run 4x 7 mana creatures without bricking hands on a regular basis. Oliphant is in an odd spot. Aggro decks can get wide, but do so early and don't want to still be playing at 7 mana. This could change in the future if 5 and 6 mana cards get wide and are good on their own (looking at you Amazons).
Excellent card and fun to play. No confusion means you can make the most of his mechanics. Farm apple bois. Get bigger. Farm more apple bois. Backline means they have to kill your Vibrant Fruits before they can attack it with creatures.
Decent card. Kills your own creatures though, so that sucks. He's also an easy target for hard removal spells (of which there are tons).
Only included because it's the only other 8-mana nature card. Maybe if he summoned two random nature creatures (>4 mana). Very underwhelming for 8 mana. Compare him to a 6/10 Cyclops Defender you can play at 7 mana (four turns earlier).
There's only one, so I suppose that makes the drunk best in slot. Of course, at 9 mana, people run Polyhymnia, but we're just talking about nature creatures. With 7 random cards, the odds are one is good, and they are only 1 mana each. Decent board clear, too.
I hope this is helpful. Yes, balance passes can change this, but understanding why the cards are tiered the way they are should give you the tools to rerank them on your own. Smart people can also disagree on parts of this tier list, so if you have some nits to pick, feel free to leave a comment!