I also have to add another vote for Apocalypse Leo - though I didnt mind most of it, I just didn't like the weird proportions. I was reminded too much of the Goombas from the Mario movie
*shudders*
But aside from that, I think the vast majority of Nick's design work has been excellent. The Space arc stuff especially was largely superb.
That said,
* April Derp was a definite weird one. Wasn't fond of that design at all.
* Bigfoot just looked off (in fact a lot of A Foot too Big's animation seemed slightly off - like a lot of it was rushed)
* The Finger was just a bizarre villain anyway, but I wasn't sold on the design. Though to be fair I think the concept as a whole was pretty flawed.
* Spiderbytez I have mixed feelings on. I dont care for the design much, but it was quite an innovative way to do a spider mutant - certainly a very unique design and I have to give them credit for that.
* I really didn't like the Mega shredder much, it was just too much of a messy mishmash, though I appreciated the little godzilla nods.
* I'm also not sure about some of the decisions on the Kraang biodroids. Buttcannons was one thing, chest buzzsaws (and I choose my words diplomatically) was quite another.