Generally, regarding the comparison of creatures, see above - armies should be compared, because the main perk of the hydra disappears in a duel, phoenixes don't rule either because their strength lies in numbers, and there's a difference whether the archangel's morale affects only itself or its entire army. And if you start comparing blindly, then that's how it should end.

I disagree. This thread is about comparing creatures, and if you want to compare armies, then create a thread about comparing armies (of course, searching for a similar one first so as not to create another clone). Here, specifically level 7 creatures are being discussed. Of course, some bonuses of certain creatures are lost in the process... but what can you do, that's the topic. In this case (archangel vs. ancient beholder), the ancient beholder doesn't lose anything, it seems. So it's not to their detriment.
But even though we are comparing creatures here, we should compare them intelligently-->
i.e., by maximizing the use of the creatures' characteristics in battle.

For now, I'll summarize the conclusion I've reached for myself (though of course I hope it will be changed into a more accurate form, as has already happened :) )
Here is the conclusion:
The strongest creature in a 1 on 1 battle for me remains the Archangel, but in a match with an ancient beholder, a draw will be recorded due to the reluctance of each creature to perish :). And I give first place to the Archangel based on an additional metric: 1 on 1 battles with other monsters. Based on that, I don't see anyone else who could beat an archangel, while ancient beholders lose to titans (although I haven't checked this version, but until I do, I'll rely on the statements of forum members).
It's something like a national football championship: everyone plays everyone, and whoever has more points wins :)

Of course, I would be grateful if someone could suggest a more accurate way to compare level 7 monsters in a 1 on 1 duel.

Added after 39 minutes

well, I've conducted several battles... 10 on 10... and even taking into account that the archangels wait and the beholders approach and the archs hit afterwards, they often lose... And a repeat turn is such an unpredictable thing... for beholders, it happened even more often for me than for archangels... and if using auto-battle, the archs didn't win a single time... but generally, the poll is about which creature is the best, not the strongest =) Ancient beholders have 80% defense reduction... meaning even the defense that a hero gives to any creature... and that helps a lot!!!! And that's why I consider the ancient beholder to be the BEST creature!!! Because for every 10 defense gained by the hero, the creature they attack uses only 2!!!

I disagree (or I don't understand) on several points:
1)>And a repeat turn is such an unpredictable thing... for beholders, it happened even more often for me than for archangels
How did you conduct the battle? Because as far as I remember, a repeat turn only occurs if a creature's morale is greater than 0. And the beholder's parameter for this is such that a repeat turn never happens :)
2)>well, I've conducted several battles... 10 on 10... and even taking into account that the archangels wait and the beholders approach and the archs hit afterwards, they often lose
Have you tried conducting a battle where the archangels are not standing in one clump? I haven't checked it myself, of course, but such a course of battle would be interesting to me from the perspective of results. Because then there will be very, very many options :)
3)>but generally, the poll is about which creature is the best, not the strongest =)
He-he, in that case, one must consider the creature as useful throughout all stages of the game :) And that is very difficult, and seems to me impossible. A lot of nuances come to light.
For example, in a game of Stronghold against Conflux.
there could be such a scenario. The Stronghold hero stands at a Conflux castle (any). The Conflux hero moves into this castle, leaves the creatures there except for 1 phoenix, puts on artifacts that maximize magic power, attacks the Stronghold hero, uses Armageddon, and capitulates. Buys back the hero in the tavern and attacks again. Then, when the Stronghold hero is sufficiently weakened, we attack with all troops and win? Which creature is better? Of course the phoenix, thanks to it, a whole army is defeated without effort.
Or there could be this scenario: Attacked with one phoenix, but the enemy has Shackles of War, and the hero is lost. Which creature is better? Beholder! And the phoenix is a bad creature overall, it ruined the hero.
Many, many nuances :) every battle, every game introduces many corrections. And it's impossible to evaluate which creature is better. Each has its own pros and cons. Every creature in a specific game can be the best, or it can not be.

Generally, it would be very good if the author of the thread explained himself what he meant :D