In terms of the worst model of all time to assemble, it's a close contest between the Black Coach, and the sixth edition metal Empire Steam Tank. Both of them just would not stay together, and the parts didn't fit properly either.
As for the ugliest models, for Fantasy, Nagash is clearly streets ahead of any model I've ever owned, while in 40K, the metal Chaos Possessed Space Marines which were released for third/fourth edition were absolutely hideous.