Everyone votes for the strongest units and forgets about the usefulness of a unit in army-vs-army battles. This is where you need to think about what makes Dragons and Titans good, considering they only excel in combat, and offer no other benefits.
The Ghost Dragon might be the weakest at its level, but if it casts "Aging" (and it does so often), that can be very powerful in large battles... The Titan, for example, had 300 health, and now it has 150; that's really useful.
Or the Hydra, which doesn't stand out at its level, but can attack all enemies around it without retaliation; that can also be very useful.
And what about the Archdevil? Reducing enemy luck by 1; that can also be very useful in large battles.
In general, dear forum members, I want to draw your attention to the fact that the question is: "Which level 7 unit is the best?", not "Which level 7 unit is the strongest."
Therefore, I vote for a truly useful and strong unit – the Archangel (resurrection and +1 to morale)