Which shouldn't matter, since you can use all of them if you want. Especially Tiamat will be a sub-Eidolon for everyone who doesn't whale, due to having the highest stats of all event Eidolons. At least for the next year.

If you're fighting Fire, nothing can beat Jack Frost, really. But if you want damage over survival, Tiamat and Reiki are about equal. Tiamat is probably slightly ahead, but less reliable and doesn't work on skills.