Which level 7 creature is the best, and why?

Archangels are unmatched in 1v1 battles among level 7 units...

I agree with that, but there were also instances where the outcome was reversed when facing a Demon in a 1v1 battle, although it was rare. This is because the Demon has 80% chance to bypass the enemy's defense and deals 300 damage. I'm stating this as it is, without exaggerating anything.

Added 5 minutes later
In a 1v1 scenario, I would give second place to the Archdevil with its unblockable attack, as it can defeat both Titans and Dragons. In general, the Titan is also good, but the Black Dragon is definitely better because it has a hatred for the Titan, which allows it to deal more damage. It also has a ranged attack and a lot of health, but in melee combat, the Archdevil is superior.