I would put a vote on the kroot, but the overall fail in design for me would be given to ....
Actually come to think of it, I really don't have too bad experiences with GW models.
Their win design goes to Gretchin. It is super ridiculous. Glue on head, Glue on base and done!
Some of my least liked models are Kroot, the Assault on black reach NOBZ (never liked the design compared to the regular kit. I find it very unlikely that an ork would keep its choppa at a 90 degree angle all the time in a battle, as well as move shoot and assault), Eldar Walker, Space Marine scouts w/bolter and Chain sword (snipers are cool), The Space Marine Assault Squad with metal Backpack (gee, I wonder where the center of gravity is between a 5 gram plastic miniature and a 10 g metal backpack up high. In game: having to use the wobbly model syndrome rule and replacing the mini with a coin, on flat ground) and the old Space Marine Chaplain.
'Orkses is never beaten in battle. If we win we win, if we die so it don't count as beat. If we runs for it we don't die neither, so we can always come back for anuvver go, see!'
This is Sebastian.