Edit: I'm really sorry for switching tier 8 10 and tier 8 25. They are mislabeled in the pictures: the left one is actually tier 8 25 and the right one tier 8 10. Thanks for the hint, twiggle.
My personal favorite is the green Naxxramas tier, whereof I find tier 3 slightly more beautiful than tier 7 25.
Tier 9 is not shown in the pictures and not included in the poll, but it's ugly anyway.