Which level 7 creature is the best, and why?

If you're lucky, it's possible, but only when the first hit of the dragon is close to its maximum, and the rest are <=43.

And if we also consider that the Ghost Dragon can deal a maximum of 36 damage to the Archangel, while the Archangel consistently deals 82 damage to the Ghost Dragon (the Ghost Dragon only manages to deal 2 hits: 1 in retaliation and 1 attacking hit), then the conclusion here is obvious.