Seen at THAT:

Tamaki is slowly stealing the show away from the magnificent Miya Miya (a very freaking strong version of a vicious tsundere) with her sudden strange face making. I am definitely having a very huge dilemma whether Tamaki is better or Miya Miya is better. Both have their merits, and it gives me a terrible headache just to choose between the two. Can I really have them both?

Quite so, and it was obvious after the ep.3.

Still, I'd go with Tama, if only because Miya-miya's real identity is unclear and ultimately Impz may be wrong. A tsundere is fundamentally nice, just badly mannered. But Miya-miya is truly half and half, in and yan. She cannot decypher who she is herself. Yandere, perhaps? Or a psychopath who defies classification?

UPDATE: Impz's commenter Warpshadow agrees:

I hate to sound like a know it all but Miya Miya isn’t a Tsundere. She is actually one of the new group of moe girls known as Yandere which is a reverse tsundere. An analogy would be a Tsundere would be like a piece of candy that is sour on the outside but sweet on the inside were a Yandere is a piece of candy that is sweet on the outside and has a razor blade on the inside. [Emphasis mine — Author]

Now you see why I avoid using "moe" on the blog. If we start including yandere into moe, moe loses all meaning, except "a sexual fetish" (in this case, fetish of mazochists (on second thought, Miya-miya always dotes over Dai (which may be not very realistic (is this a blog or a LISP program?)))).