Ein : terrible fishnet texture, boring color for the pants, unmemorable design overall.
Momiji : does not follow the original concept which was cool, hate the green accessories on it though.
By now there are too many to count. XD
But Ein's C4 is a good contender. The design makes so sense/follows no theme, it's dull and the color combination of mustard and brown is the ugliest ever.
Because of clipping... Kasumi's first Halloween costume 26; it is my favorite if she is not long hair. I don't even use Kasumi's long hair I just want to with that one haha. Also Ein's destroyed costume 11. That is an inaccurate representation of Zeno's destroyed armor in favor of reappropriating EIn's UGLY C4 with black pants. Literally a recolor of Ein's worst item (no rag).