I would start with the human head/neck combination. An intelligent designer would not have made our heads to as proportionally large in comparison to our necks: whiplash is evident.
most all other mammals (biologists correct me if I'm wrong) have a head that is equal to or smaller in size compared to their neck. Human beings being bi-ped, this makes it even worse.
We can also look at cancer: our own bodies attack us. Our own cells attack us. I'm not too familiar with cellular bio, as much as someone like WK, so I don't know what else I could say to expound on this.
These two are all I can think of with even half an argument from myself, so I'll leave it there.
Who are we? We find that we live on an insignificant planet of a humdrum star lost in a galaxy tucked away in some forgotten corner of a universe in which there are far more galaxies than people
-Carl Sagan
For me, it is far better to grasp the Universe as it really is than to persist in delusion, however satisfying and reassuring.
-Carl Sagan