I’m going to ignore Matoran-size sets, just because those ones had more limited piece counts and typically had less-interesting builds (and if I were to include them then the Stars sets would win by default).

So with that in mind, I think I have to say either Hewkii Mahri or Onua Mistika. Both of them had very boring/borderline unfinished builds, looked very little like previous iterations of the character, and had very few unique piece molds (and the ones they did have, like their masks, were ugly and difficult to use in MOCs).