Oh a really difficult question.
I reckon you mean rather Monsters than Summoners?
On what I think of the ranks of summoners I wrote a post before the recent new cards. With the new dragons Selenia got even stronger.
The new Summoner with +1 tempo looks pretty interesting. I have no idea yet where I would put her on the ranking.

I get why you like Cocatrice :D. Cheap, neutral and flying+dodge are really tricky.

It is more difficult for me to assess the cards with max level.
Cocatrice on lvl 1 against lvl4 often is not so strong.
The Goblin Sorcerer with magic and sneak I sometimes found massive.
Spineback Turtle on lvl1 has immense stats. also thorns are really cool, serpentine soldier has them too.

BUT. As I rank pure magic deck still very high, I deem Haunted Spirit currently as the best card. Magic reflect+void is huge and then 10 Life+ heal + 4 melee attack is also pretty decent for a tank. And for a tank with such much life tempo necessarily doesn't matter too much.
Maybe that Lord of darkness guy is nearly similarly strong though. Still. The magic :D.

Maybe the Gold Dragon or Shin-Lo (1 Mana!) is the best in the end :D.
Those to get will be sooo expensive.