In this post, I would like to show some my findings and see which cards have the most stats for each mana. If i have missed something, please write in comments.

For 1 mana, the theoretical best card would be either 10/10 hyperion(mythic card), or an 8/8 Octavia, Manashard Queen, which however cannot be played on the first turn, because it comes from Deuteria, Manashard Mage

[IMAGE: https://files.peakd.com/file/peakd-hive/bosfik/23uFPgJsHDrp6McHg3tXyRaPPpZwGXyghoRbCwwijC5BjhjCNdxDfVxeFttgezZPu7vrt.png]

If we don't count these cards, Peculiar Spellwyrm with whopping 2/6 mana is the best stats for one mana. Having quite a bad, but not the worst debuff, I think that card is quite good, at least for its manacost.
[IMAGE: https://files.peakd.com/file/peakd-hive/bosfik/23tmKruLmJH71xswmaNo9Ttk1R6XPgxT7SrKHxAMBGVC1XVwKtHXjGrAcSZrJB8RK68k4.png]

For 2 mana, everything is less interesting(ignoring PW), it's either a 4/3 Void Banshee:
[IMAGE: https://files.peakd.com/file/peakd-hive/bosfik/23wMY9rCrU3zENQEMdatMFDzm7X9iarqtMHsgPgRhoUMCfBCdukXN26RUWwmNGN28bj6D.png]

or a 3/6 Wasteful Shax with a very bad debuff:

[IMAGE: https://files.peakd.com/file/peakd-hive/bosfik/23x159k2kPnVhT7TaNgzDDB2Eb8H86Kze8Qf1Ato1Uk6jBwSdGVpvFGGGAqU9DinS7bxU.png]

3 mana is even less interesting, with Greed Banestrider being the best card with only 4/4 stats.

[IMAGE: https://files.peakd.com/file/peakd-hive/bosfik/23w2ktqGikxwDRegVsSFrpfnoZbLLTrXK7LRtM3N5v86xonn8HdBnjxfq6ggKQHSRhFs7.png]

There is also a 7/2 card, but it summons quite a strong creature for your opponent:

[IMAGE: https://files.peakd.com/file/peakd-hive/bosfik/23tmQ5odhd7M8DSuRhkqsrxio6nj96PRE8RLxXveWNXCTdJfmSa64sQRvaJzNb8qVXFYx.png]

4 mana is also quite uninteresting with a 6/5 Crimson Oni:

[IMAGE: https://files.peakd.com/file/peakd-hive/bosfik/23w2kgoePByShaLQ1Yo2qvhjDECxfc6BafmdWv4eK3FVBWbVtYnhHDY78WPe4zxyw7HW2.png]

Moon frenzied cyclope can also get very high stats, but i won't count him because even if you are frenzied, it only gets to 5/5 on the end of first turn.

Same with 5 mana -
[IMAGE: https://files.peakd.com/file/peakd-hive/bosfik/23uQvpPZdKKRRf9cguZ1vAvpyK5BTynmqQjkGbSFVnRx7KU1pbXBCc3YZyQxKbaQvra1g.png]

But on 6 mana the interesting part starts

We have an 8/10 Spiral golem with a nice text and another mythic - Prometheus, which has burn 3 but 10/10 stats:

[IMAGE: https://files.peakd.com/file/peakd-hive/bosfik/23wMYatMoy5wy34PkmLW4p3Zhqy487eKvkqt5Uc9jmkLgah3nYwQcLPw5pdvQ3FUZp9iN.png]

[IMAGE: https://files.peakd.com/file/peakd-hive/bosfik/Eoc8xHpBTyvvh2ChXHdSQ5d3RJ3TqKCiwonFRcGZ45Vrz9qcVRFe4c1kPjYLzG3kkrQ.png]

7 mana has a 10/10 burnished bull without any stats and yet another mythic card - Atlas

[IMAGE: https://files.peakd.com/file/peakd-hive/bosfik/EoptPrAeMVJ6aHwQ3rvMU2NfZz88efbPbXypc6QgPP16KaMQACAsFJ9YVP3B7QdiVNg.png]

[IMAGE: https://files.peakd.com/file/peakd-hive/bosfik/23w2sLxTAB2jsemCV72Yd8XMYvmx5xrQSDn6cuWojTTFLnjo6g1qtKdtNS1y8B2hkdbPJ.png]

For 8 mana we have a boss, Guardian of the Underworld with absurd 13/17 stats, but a terrible roar

[IMAGE: https://files.peakd.com/file/peakd-hive/bosfik/23uFKdbPQA4YqGEEzg6MbZsnLpBPUakWk1w891eCu25bWHpxPPR5PTDkMm1K35KQjeZMN.png]

And the last but not least - 16/16 Arius for 9 mana

[IMAGE: https://files.peakd.com/file/peakd-hive/bosfik/23uFTcrZZzWjLJ6NE7VbVEmnvHY87oAWFmpR5apE9o1ALTsCWSiZyNmwN6k9mUQSFKi4p.png]

There is also one card which gives a creature strength and health based on the sum of atributes in the void, but I couldn't find its name, so decided to mention it here. If you have any other ideas for posts connected with cards, like maybe try to find the weakest one for each mana, feel free to write down here!

P.S. the best stats doesn't mean the best cards - in fact all of these creatures can be easily removed with ratify, so this post is just about pure stats.