Which level 7 creature is the best, and why?

That's right. But for the sake of completeness, it should be 10x10. In a 1x1 battle, the speed parameter wins, which isn't very interesting.

So what will you prove with that? Why only 10x10? Make it 10x10, 100x100, and 1000x1000, and then see who wins overall and draw conclusions based on that... And the turn priority doesn't matter here; all monsters of level 7 have different speeds, except for the Behemoth and Hydra, but who will win is not even a question...