There have actually been several discussions on this previously, where people have attempted to extrapolate mathematically which creature is the best.
The most notable one i can think of recently was extremely controversial. The main difficulty, I believe is in agreeing on the relative value of different traits.
Arcane Wonders probably has an agreed upon model for this, but we players have had a hard time figuring it out with the limited data available to us. Especially since you can't judge a creature in a vacuum. The value of it's traits are largely determined by what cards are used to support it and what cards are used to combat it.
I think if you want to start a legitimate discussion on this subject, and not just an opinion piece, we should start by clearly defining the topic of conversation. You mention dividing creatures into three classes based on level, however you have overlap with level 4 creatures being both mid ranged and fatties. What distinguishes it from one or the other? You then ask which creature is best, and I would really really need to know what you mean by that before I could give an answer. Do you mean which one has the most damage output? The hardest to kill? The most versatile? Are you measuring this against their mana cost? Which traits do you consider better and why?
That last question really deserves a topic all to itself. My point is, you have asked a really vague and open ended question that lacks agreed upon figures with which to evaluate the responses. It is mostly going to be pure opinion unless you redefine the discussion and setup rules for us to operate under.
