I think the worst kit I've come across is a Kopro Skoda D.1 in 1/72 scale.
The 4 struts of the parasoll-wing where all of different lenght and width, you had to wittle something resembling a propeller from a lump of plastic, and the large decals turned instantly into a jigsaw-puzzle when introduced to water...
I persevered though, and the end result was ok, at least as it's an unusual maschine in an unudual scheme.
(There is a few more pictures on my blog, the text are in Norwegian though https://polarcoordinate.wordpress.com/2013/05/19/ferdigbygget-skoda-d-1-kopro-172/)