Which level 7 creature is the best, and why?

In a 100 vs. 100 battle, speed is even more important than in a 1 vs. 1 fight. Whoever strikes first will immediately kill a certain number of monsters, meaning the counterattack will be with fewer than one hundred units. In a 100 vs. 100 battle, Phoenixes will definitely dominate, unless you take 200 of them (because their growth rate is twice as high).