Which level 7 creature is the best, and why?

All of this is very easy to verify (of course, without magic or any artifacts): create a small area of land in the editor (it must be sand!) and place monsters of level 7, for example, in groups of 1, 10, 100, and 1000; set the hero's defense and attack to 0, and let the battle begin! In all these cases, the archangels won! The only thing to consider is that phoenixes have twice the growth rate, and if you place twice as many of them, they will defeat everyone. You can, of course, check all this yourself if you're not too lazy... But personally, I like the monsters the most; it's just a personal preference.